In a standard API-led connectivity model, the First Tier (Experience APIs) is responsible for tailoring data for specific systems of engagement.
The Experience APIs take the core data from the lower tiers and transform it into the specific return formats (e.g., JSON for mobile, XML for legacy web) and security protocols (e.g., OAuth for Salesforce, API Keys for web) required by each consumer. Option B correctly identifies that these transformations and security enforcements should happen at this outer layer. While an API Gateway (Option A) can provide generic security and rate limiting, it is the Experience API layer that provides the functional transformation and specific protocol requirements defined by the business needs of the engagement systems.

When designing integrations that specifically involve Chatter, social collaboration, or Experience Cloud sites, the Connect REST API (formerly known as the Chatter REST API) is the architecturally recommended choice. While the standard REST API is optimized for CRUD operations on data records, the Connect REST API is specifically designed to handle the complex, nested structures of social feeds, including posts, comments, hashtags, and mentions.
The Connect REST API provides a specialized resource for feed elements that simplifies the process of identifying specific social markers like hashtags. In this use case, the mobile app or a middleware service can subscribe to or query the feed. The Connect REST API returns structured data where hashtags are identified as distinct message segments, making it trivial for an application logic to detect the #thanksNTO tag and trigger a subsequent call to the Twitter API.
From an architectural standpoint, using the Connect REST API offers several advantages over the standard REST API or Streaming API for this requirement:
Efficiency: Connect REST API responses are structured specifically for presentation in mobile applications, providing only the relevant social metadata and localized content needed for the feed.
Feature Richness: It provides native support for social actions such as liking, sharing, and following, which are often required alongside feed monitoring.
Scalability: It is designed to handle the high-volume, real-time nature of social interactions within Experience Cloud and mobile ecosystems.
While the Streaming API (Option B) can notify an application of record changes, it does not provide the rich, formatted social context that the Connect REST API delivers. The standard REST API (Option A) could technically access FeedItem records, but it would require significant custom parsing logic to identify hashtags within the raw body text, whereas the Connect REST API handles this segmenting natively. Therefore, for building custom mobile experiences that interact with Chatter data, the Connect REST API is the superior solution.

When a custom mobile application already secured with OAuth needs to update a record in Salesforce, the standard architectural recommendation is to use the REST API. The REST API is optimized for mobile environments because it uses lightweight JSON payloads and follows standard HTTP methods (such as PATCH for updates), which are highly compatible with mobile development frameworks.
In this specific scenario, the architect must address the Field-Level Security (FLS) constraint. Because the geolocation field is set to read-only for users, a standard UI-based update would typically fail. However, when using an inbound REST API call with a properly authorized integration user or via a "System Mode" context (if utilizing a custom Apex REST resource), the system can be configured to bypass UI-level restrictions while maintaining data integrity.
The mobile device captures the coordinates via the device's native GPS capabilities and initiates an inbound call to the Salesforce REST endpoint. Option A (Apex inbound call) is a subset of REST functionality but is only necessary if complex server-side logic is required that the standard REST API cannot handle. Option C is technically incorrect as mobile devices do not typically "receive" callouts from Salesforce in this pattern; they initiate the requests. By leveraging the standard REST API, the architect ensures a scalable, secure, and standardized integration that adheres to Salesforce's mobile-first integration principles.

The evaluation of an integration landscape is a process of rationalization. The goal is to identify which legacy systems Salesforce will replace (System Retirement) and which systems it must coexist with (Integration).
In this scenario, Salesforce is being implemented for Case Management. Salesforce Service Cloud is the industry leader for this specific function. Therefore, the legacy Case Management System should be retired. Any architecture that suggests "integrating" Salesforce with the legacy Case Management system (Options A and B) is creating a redundant and complex "dual-master" scenario that increases technical debt.
To provide a successful support experience, Salesforce needs to be the central "Engagement Layer," which requires integration with the remaining ecosystem:
Email Management System: To support "Email-to-Case" and ensure all customer communications are captured within the Salesforce Case record.
Order Management System (OMS): Support agents often need to verify purchase history or shipping status to resolve a case. A "Data Virtualization" or "Request-Reply" integration with the OMS is vital.
Data Warehouse: For long-term historical reporting and cross-functional analytics, Salesforce must push case data to the enterprise Data Warehouse.
By evaluating the integration with the Data Warehouse, Order Management, and Email Management systems, the architect ensures that Salesforce is enriched with the context it needs to resolve cases while simultaneously retiring the redundant legacy support system.