It has been raining _____ summer was over.

Answer: Since

Explanation: "Since" indicates the starting point of an action that continues up to the present. The sentence "It has been raining since summer was over" means the rain began when summer ended and has continued until now. (A slightly more natural phrasing is "since summer ended".)

Why the other choices are wrong:

• "For" is used for durations (e.g., "for two hours"), not to mark the starting point in time.

• "Until" implies the action continued up to that point and then stopped (e.g., "It rained until summer was over"), which conflicts with the idea of it still raining.

• "About" expresses approximation and does not indicate a specific starting point in time.