A stored response carries Cache-Control: max-age=600 and a Last-Modified date two years old. May a cache use a Last-Modified heuristic to extend its freshness lifetime?
No, unless the status code is defined as heuristically cacheable
Yes, but only for a shared cache
Yes, heuristics apply whenever a validator is present
No, a cache must not use heuristics when an explicit expiration time is present in the stored response✓Correct answer
Explanation
Heuristic freshness is a fallback for responses that carry no explicit expiration at all; an explicit max-age settles the question and a MUST NOT forbids overriding it. The heuristically cacheable status code list matters only in the case where explicit freshness is absent.