An Application Calls a Service Run by a Vendor

An application calls a service run by a vendor. The vendor charges based on the number of calls. The finance department needs to know the number of calls that are made to the service to validate the billing statements. How can a solutions architect design a system to durably store the number of calls without requiring changes to the application?

A) Call the service through an internet gateway.
B) Decouple the application from the service with an Amazon Simple Queue Service (Amazon SQS) queue.
C) Publish a custom Amazon CloudWatch metric that counts calls to the service.
D) Call the service through a VPC peering connection.