What did you do?
This is a sentence.

The correct answer is 'True'.

A complete sentence is a sentence that has a subject, a verb, and an object. It conveys a complete thought. 'What did you do' here, it has a question word 'what' and past tense verb 'did', subject 'you' and present tense verb 'do'. It is an interrogative sentence and so it has this structure. So, it is a complete sentence. The above statement is true.