Concept:The phrase "let off the hook" is an informal idiom meaning to release someone from blame, punishment, or an unpleasant obligation.
Explanation:The idiom comes from the image of a fish being removed from a hook and set free.
When a person is "let off the hook," they are spared from a duty, a difficulty, or a penalty.
In the given sentence, Tom was "let off the hook."
This means that Tom escaped the punishment that was expected.
The speaker's surprise shows that Tom was not punished, although he should have been.
Option A, "not been punished," is the only interpretation that matches this meaning.
Options B, C, and D are unrelated to the idiom.
Answer:A. not been punished