Implicit sharing will automatically handle this sharing requirement with standard functionality, as it grants read-only access to parent accounts when users have access to child opportunities. This is also known as account-opportunity sharing3. Creating a criteria-based sharing rule (option A) or an owner-based sharing rule (option D) is not necessary, as they are used to grant additional access based on record criteria or ownership.
Adding appropriate users to the account team (option C) is also not required, as it is used to grant access to specific users or groups for individual accounts.

The best solution to comply with the DR policy is to leverage a 3rd party tool that extract Salesforce data/metadata and stores the information in an external protected system. This solution can help create a backup of Salesforce data and metadata in case of a disaster or data loss event. It can also help restore data from the backup system to Salesforce if needed. There are various 3rd party tools available in the AppExchange or online that offer data backup and recovery services for Salesforce67. Leverage Salesforce weekly exports and store data in flat files on a protected system is not a good solution because it does not include metadata backup and it does not allow granular or automated data recovery. Utilize an ETL tool to migrate data to an on-premise archive solution is not a good solution because it does not include metadata backup and it may require complex data transformations and synchronizations. Write a custom batch job to extract data changes nightly and store in an external protected system is not a good solution because it does not include metadata backup and it may have performance or reliability issues.

According to the Salesforce documentation, a relationship field is a field that allows linking one object to another object in Salesforce. There are different types of relationship fields that have different characteristics and behaviors, such as master-detail, lookup, indirect lookup, external lookup, etc.
To recommend a type of relationship field for this scenario, where legacy inventory data will be stored in a custom child object called Inventory__c, inventory data should be related to the standard Account object, the Inventory__c object should inherit the same sharing rules as the Account object, and anytime an Account record is deleted in Salesforce, the related Inventory__c record(s) should be deleted as well, a data architect should recommend:
Master-detail relationship field on Inventory__c, related to Account (option B). This means creating a field on the Inventory__c object that references the Account object as its parent. A master-detail relationship field establishes a parent-child relationship between two objects, where the parent object controls certain behaviors of the child object. For example, a master-detail relationship field can:
Inherit the sharing and security settings from the parent object to the child object. This means that the users who can access and edit the parent record can also access and edit the related child records.
Cascade delete from the parent object to the child object. This means that when a parent record is deleted, all the related child records are also deleted.
Roll up summary fields from the child object to the parent object. This means that the parent object can display aggregated information from the child records, such as count, sum, min, max, or average.
Master-detail relationship field on Account, related to Inventory__c (option A) is not a good solution, as it reverses the direction of the relationship. This means creating a field on the Account object that references the Inventory__c object as its parent. This is not possible, as a standard object cannot be on the detail side of a master-detail relationship. Indirect lookup relationship field on Account, related to Inventory__c (option C) is also not a good solution, as it is a special type of relationship field that allows linking a custom object to a standard object on an external system using an indirect reference. This is not applicable for this scenario, as both objects are in Salesforce and do not need an external reference. Lookup relationship field on Inventory__c related to Account (option D) is also not a good solution, as it establishes a looser relationship between two objects than a master-detail relationship. A lookup relationship field does not inherit sharing and security settings from the parent object to the child object, does not cascade delete from the parent object to the child object, and does not roll up summary fields from the child object to the parent object.

The correct answer is C and D. To minimize performance and storage issues, a data architect should recommend leveraging on premise data archival and building integration to view archived data, and leveraging big object to archive case data and lightning components to show archived data. These solutions will allow some user groups to access historical cases for up to 7 years without consuming too much storage space or affecting the performance of queries and reports. Option A is incorrect because exporting data out of salesforce and storing it in flat files on external system will make it difficult to access and query the data.
Option B is incorrect because creating a custom object to store case history and run reports on it will still consume a lot of storage space and impact the performance of queries and reports.