An external application calls a Pega REST service, which takes a significant amount of time...

 Service Request Processor for Asynchronous Processing:

Configuring a REST service to run asynchronously using a service request processor ensures that the service runs without making the external application wait. The system returns a unique identifier instantly and processes the request in the background.

 References:

Pega's best practices for REST services and asynchronous processing recommend using service request processors for handling long-running processes.

 Therefore, the correct answer is:

B. A REST service that runs asynchronously by using a service request processor.