Final answer:
The correct verb to fill in the blank is 'bought', making the complete sentence read as 'They bought a car last month'. This is due to the sentence referring to a past event, therefore requiring a verb in the past tense.
Step-by-step explanation:
In the sentence 'They _____ a car last month', the correct verb to use would be 'bought'. This is because the sentence is referring to a past event, and 'bought' is the past tense of the verb 'buy'. So the correct sentence should be, 'They bought a car last month'.
Learn more about Past Tense Verbs