In the following sentence, choose the option that improves the underlined part: He thanked me for what I (have done) for his wife.

Introduction / Context:
This grammar question tests the correct use of tense in a complex sentence involving reported appreciation. The sentence describes someone thanking another person for a past action. English often uses the past perfect tense to show that one past event happened before another past event. Recognising this relationship helps in choosing the correct tense for the part in brackets.

Given Data / Assumptions:
- Full sentence: He thanked me for what I have done for his wife.
- Underlined or bracketed part: have done.
- We must select the option that makes the sentence grammatically correct and natural.
- The main verb thanked is in the simple past.

Concept / Approach:
When we talk about two actions in the past where one happened earlier than the other, it is common to use the past perfect tense for the earlier action. Here, I did something for his wife first, and later he thanked me for it. Therefore, the action of doing came before the action of thanking. The correct tense for the earlier action is had done, not have done. Present perfect have done would link the action closely to the present moment, which does not fit with the past context of thanked.

Step-by-Step Solution:
Step 1: Identify the main clause and its tense. He thanked me is in the simple past.Step 2: Notice that the action I did for his wife happened before he thanked me, so it is an earlier past event.Step 3: The correct way to show that one past action occurred before another past action is to use the past perfect tense for the earlier action, that is, had done.Step 4: Replace have done with had done to form He thanked me for what I had done for his wife.Step 5: Check that the sentence now sounds natural and shows the correct time relationship between the two actions.

Verification / Alternative check:
If we keep have done, the sentence reads He thanked me for what I have done for his wife, which mixes simple past and present perfect without a clear reason. Present perfect is usually used when the earlier action remains relevant up to the present or when the time is not specified. Since the thanking already took place in the past, it is more accurate to see the helpful action as completed before that moment. The version with had done matches the standard pattern: He thanked me for what I had done for his wife, similar to He thanked me for what I had done earlier.

Why Other Options Are Wrong:
- Had been done uses passive voice and would imply that the action was done by someone unnamed, not by me, which changes the meaning.
- Have been done also uses passive voice and the present perfect tense, which is inappropriate in this fully past context.
- No improvement would keep have done, which is not the best tense choice given the clear sequence of past actions.

Common Pitfalls:
Students sometimes overuse the present perfect tense because they see it frequently in examples like I have finished my work. They may also forget the function of the past perfect in showing earlier past actions. A helpful strategy is to place time markers mentally: first the helpful action, then the expression of thanks. Whenever there is a clear earlier past followed by a later past, the earlier one usually needs had plus past participle. Practising more such pairs strengthens this understanding.

Final Answer:
The correct improvement is had done, so the sentence should read: He thanked me for what I had done for his wife.