Which HTTP response header indicates that authentication is required?

The Authorization header is a crucial component in HTTP communications, as it is specifically designed to provide credentials for authentication. When a server requires a client to authenticate, it may respond with a 401 Unauthorized status code, along with an Authorization header indicating what authentication methods are supported and prompting the client to provide the necessary credentials. This mechanism is a fundamental part of RESTful API security, allowing servers to protect resources from unauthorized access.

In contrast, the other headers mentioned serve different purposes. Cache-Control dictates how caching mechanisms behave, Content-Type informs the client about the type of data contained in the response body, and Accept-Charset specifies the character sets that the client is willing to accept in responses. None of these headers relate to authentication requirements, which is why the Authorization header is the correct choice.