Final answer:
The correct sentence is: The mechanic did his best, but it was an old car.
Step-by-step explanation:
The correct sentence is: The mechanic did his best, but it was an old car. In this sentence, 'did his best' is the past tense of the verb 'do,' which is correct. The other options either use the wrong verb tense or incorrect forms of the verb.
Learn more about Verb Tenses