Which of the following are always included in both the request and response headers?

The correct answer is that cookies are indeed always included in both the request and response headers. In the context of web communication, cookies serve as a mechanism for maintaining stateful information between the client and server. When a client (such as a web browser) makes a request to a server, it sends any relevant cookies associated with that domain in the request headers. Subsequently, the server can include new cookies, or modify existing ones, in the response headers it sends back to the client. This interchange is critical for managing session information, tracking user behavior, and enabling personalized experiences on websites.

While connection-related information can appear in both request and response headers, it is not guaranteed to be present in every single interaction. Specifically, headers related to the connection (like Connection: keep-alive, etc.) can vary based on the specifics of the request/response cycle.

Data, in the context of the request and response headers, typically refers to the body content of the messages—not the headers themselves. This content is separate from the header fields, hence it is not consistently included in both headers.

Thus, cookies are distinctly notable for their omnipresence in both request and response headers, incorporating functionality that is essential for many web applications.