A company needs to integrate a legacy on-premise application that can only support SOAP API.

For an outbound, declarative, Fire-and-Forget integration to a legacy SOAP-based system, Salesforce Outbound Messaging is the native tool of choice. Outbound Messaging sends an XML message to a designated endpoint when specific criteria are met. It is highly reliable as Salesforce will automatically retry the delivery for up to 24 hours if the target system is unavailable.

For the communication back from the legacy system to Salesforce, a strongly-typed SOAP API approach is required. The Enterprise WSDL is the correct recommendation here because it is a strongly-typed WSDL that is specific to the organization's unique data model (including custom objects and fields). Using the Enterprise WSDL allows the legacy system to communicate with Salesforce using specific data types, providing compile-time safety and reducing errors during the mapping process.

Option A is less efficient because Platform Events would likely require middleware to translate the event into the legacy system's SOAP format. Option B suggests the Partner WSDL, which is loosely-typed and designed for developers building tools that must work across many different Salesforce orgs. Since this is an internal integration for a specific company, the Enterprise WSDL provides a much more streamlined development experience with better data integrity. By combining Outbound Messaging (for fire-and-forget delivery) and the Enterprise WSDL (for the strongly-typed callback), the architect fulfills the technical requirements while minimizing custom code.