To identify all subscribers who are unsubscribed from a specific Business Unit, the correct Data View to query is ListSubscribers (A). This Data View contains information about the subscription status of subscribers at the list level, including unsubscribe status for specific Business Units.
References:
Salesforce Marketing Cloud Data Views
ListSubscribers Data View

Before deleting a sendable data extension that contains a large number of contact records, it is important to manage the contact deletion through Contact Builder. Deleting the data extension from Contact Builder ensures that all related contact information is also removed from the contact model.
Salesforce Contact Builder

Before importing a compressed file into Marketing Cloud, a File Transfer activity is required to move and decompress the file. The File Transfer activity allows you to specify the source and destination locations and to decompress the file, making it ready for the import process.
Salesforce File Transfer Activity

Switching to Tenant Specific Endpoints (TSEs) provides the benefit of improved API performance (D).
Tenant Specific Endpoints are designed to provide optimized performance by ensuring that API calls are routed through endpoints that are dedicated to a specific tenant, reducing latency and potential congestion issues associated with shared endpoints.
References:
Salesforce Marketing Cloud Tenant Specific Endpoints
API Best Practices