An HTTP request containing which header instructs the content server to return whether the requested object has been modified since the last visit?

The correct answer is based on the functionality of the HTTP protocol, specifically how caching and conditional requests work. The "If-Modified-Since" header is used in conjunction with a GET request. When a client makes a GET request to a server while including the "If-Modified-Since" header, it effectively tells the server to return the requested resource only if it has been modified since the date and time specified in that header.

If the resource has not been modified, the server can respond with a 304 Not Modified status, indicating to the client that it can use its cached version of the resource. This mechanism is efficient because it reduces the amount of data transferred and minimizes server load, as the server doesn’t have to send the resource if it hasn't changed.

In contrast, the other options presented do not fit the purpose of the "If-Modified-Since" header as effectively:

• The POST method is generally used to submit data to the server, and using "If-Modified-Since" in this context does not align with its intended use.

• The HEAD method is designed to retrieve the headers of a resource without the actual content. While it does support conditional requests, the expected usage with "If-Modified-Since" is