Which action is the appropriate response?

A client is written that uses a REST API to interact with a server. Using HTTPS as the transport, an HTTP request is sent and received an HTTP response. The response contains the HTTP response status code: 503 Service Unavailable.
Which action is the appropriate response?
A. Add an Authorization header that supplies appropriate credentials and sends the updated request.
B. Resend the request using HTTP as the transport instead of HTTPS.
C. Add an Accept header that indicates the content types that the client understands and send the updated request.
D. Look for a Retry-After header in the response and resend the request after the amount of time indicated.