What are common HTTP caching headers and their purposes?

• A. Cache-Control, ETag, Last-Modified control caching behavior and validation.
• B. Content-Type, Content-Length, Date handle encoding only.
• C. Last-Modified, Cache-Control, ETag control caching behavior and validation.
• D. Accept, Accept-Language, Accept-Encoding manage request negotiation.

Answer: (C)

Caching relies on headers that tell caches when to store a response and when to check if it’s still fresh. The trio of Last-Modified, Cache-Control, and ETag does exactly that: Cache-Control provides the rules for how long to cache and under what conditions; ETag gives a version identifier you can use for conditional requests to revalidate a cached copy; Last-Modified supplies a timestamp you can use with If-Modified-Since to verify freshness. Together, they cover both how long to keep a response and how to determine if the cached version is up to date.

Other headers listed aren’t primarily about caching. Content-Type and Content-Length describe the data’s format and size rather than how caching should behave, and Date is simply when the response was generated. Accept, Accept-Language, and Accept-Encoding relate to what the client will accept or negotiate, not caching strategies. That's why the combination of Last-Modified, Cache-Control, and ETag best captures common caching headers and their purposes.