D. Query string
Explanation
Option A is incorrect. Payload compression is used to compress and decompress the payload to and from your API Gateway. It is not used to separate cache responses.
Option B is incorrect. Custom domain names are used to provide more readable URLs for the users of your AIPs. They are not used to separate cache responses.
Option C is incorrect. An API stage is used to create a name for your API deployments. They are used to deploy your API in an optimal way.
Option D is correct. You can use your query string parameters as part of your cache key. This allows you to separate cache responses for equity requests from fixed income request responses.