Final answer:
To transform the sentence 'He studied French yesterday' into a negative sentence, use the negative form of the auxiliary verb 'did' and add 'not' before the main verb 'study'. The correct answer is A) 'He didn't study French yesterday'.
Step-by-step explanation:
To transform the sentence 'He studied French yesterday' into a negative sentence, we need to use the negative form of the auxiliary verb 'did' and add 'not' before the main verb 'study'. So, the correct answer is A) He didn't study French yesterday.