Explanation
Option B is a correct finding, because it means that the company can use their cloud-based ETL tools to extract data from the legacy systems via the APIs exposed by the on-premise middleware, transform the data as needed, and load the data into Salesforce using queues. Queues are a way of managing asynchronous operations, such as bulk data loading, by placing them in a queue and executing them when resources are available1.
Option C is also a correct finding, because it means that the company can expose their legacy data to the cloud via their on-premise middleware APIs. APIs are a way of enabling communication and data exchange between different systems using standard protocols and formats2. By providing APIs to access their legacy data, the company can use any cloud-based tool or application that can consume those APIs to retrieve and manipulate the data.
Option A is not a correct finding, because it means that the company cannot access their legacy data from the cloud, which prevents them from using their cloud-based ETL tools or any other cloud-based integration solution. If only on-premise systems are allowed to access the legacy systems, then the company would need to use an on-premise integration tool or application to copy the data to Salesforce.
Option D is not a correct finding, because it does not affect the ability to copy legacy data in Salesforce.
Queues are useful for on-premise integration, but they are not necessary for copying data to Salesforce. The company can use other methods, such as direct API calls or batch processes, to load data into Salesforce without using queues.
References: 1: Queueable Apex | Apex Developer Guide | Salesforce Developers 2: API Basics | SOAP API Developer Guide | Salesforce Developers

Explanation
Change Data Capture Event is the best option to meet this requirement. Change Data Capture Event is a type of streaming event that notifies subscribers of changes to Salesforce records, such as creation, update, delete, and undelete operations1. By subscribing to Change Data Capture Event for the Case object, the dashboard component can receive real-time updates on the number of closed Cases. Push Topic Event is another type of streaming event that notifies subscribers of changes to Salesforce records that match a SOQL query2. However, Push Topic Event has some limitations, such as not supporting all SOQL features and not capturing delete and undelete operations3. Platform Event is a type of streaming event that delivers custom notifications within the Salesforce platform or from external sources4. Platform Event is not suitable for this requirement because it is not tied to Salesforce records and requires custom logic to publish and subscribe. Generic Event is a type of streaming event that sends custom JSON notifications to subscribers without a predefined schema5. Generic Event is not suitable for this requirement because it is not tied to Salesforce records and requires custom logic to publish and subscribe.

Answer D is valid because using Bulk API 2.0 for both inserting and querying data can meet the requirements of loading and extracting large volumes of data in a day. Bulk API 2.0 is a RESTful API that allows creating, updating, deleting, or querying millions of records asynchronously by uploading or downloading CSV or JSON files. Bulk API 2.0 has several advantages over Bulk API 1.0, such as:
It does not count against the daily limit of 5,000 batches per rolling 24-hour period.
It supports PK Chunking for queries, which automatically splits large data sets into manageable chunks based on the primary key.
It simplifies the job lifecycle and reduces the number of API calls needed to perform a bulk operation.
Answer A is not valid because using Bulk API 2.0 for inserting data and REST API for querying data is not an optimal solution for handling large volumes of data in a day. REST API is a synchronous API that allows creating, updating, deleting, or querying individual records or small batches of records using HTTP methods. REST API has several limitations and drawbacks for this use case, such as:
It counts against the daily limit of 15,000 API requests per 24-hour period.
It does not support PK Chunking for queries, which means that large data sets may exceed the query timeout or heap size limits.
It requires more API calls and processing time to perform a bulk operation than Bulk API 2.0.
Answer B is not valid because using Bulk API 1.0 for both inserting and querying data is not a feasible solution for handling large volumes of data in a day. Bulk API 1.0 is a SOAP-based API that allows creating, updating, deleting, or querying millions of records asynchronously by uploading or downloading XML or CSV files. Bulk API 1.0 has several limitations and drawbacks compared to Bulk API 2.0, such as:
It counts against the daily limit of 5,000 batches per rolling 24-hour period, which may not be enough to load and extract 90M and 30M records respectively.
It does not support JSON format for data files, which may not be compatible with some external systems or applications.
It requires more API calls and complexity to manage the job lifecycle and handle errors or retries than Bulk API 2.0.
Answer C is not valid because using Bulk API 1.0 for inserting data and REST API for querying data is not a suitable or reliable solution for handling large volumes of data in a day. As explained above, both Bulk API 1.0 and REST API have limitations and drawbacks that may affect the performance, efficiency, and scalability of the integration.

https://developer.salesforce.com/docs/atlas.en-us.platform_events.meta/platform_events/platform_events_api_considerations.htm

Explanation
Both Platform Events and Outbound Message offer declarative means for asynchronous near-real time needs.
They aren't best suited for real-time integrations (A). In both Platform Events and Outbound Messages, the event messages are retried by and delivered in sequence, and only once. Salesforce ensures there is no duplicate message delivery (B). Message sequence is possible in Outbound Message but not guaranteed with Platform Events. Both offer very high reliability. Fault handling and recovery are fully handled by Salesforce
Number of concurrent subscribers to Platform Events is capped at 2,000. An Outbound Message configuration can pass only 100 notifications in a single message to a SOAP end point (D). Both Platform Events and Outbound Message are highly scalable. However, unlike Outbound Message, only Platform Events have Event Delivery and Event Publishing limits to be considered (E). Reference: Salesforce Integration Architecture Designer Resource Guide, page 23