Fill in the blank with the most appropriate word.
____ their sister given a present last year?
The normal sentence order for the interrogative is modal/auxiliary verb + subject + base form of the main verb. In the given context, the sentence is a polar (yes/no) question. Here, ‘was’ indicates the past tense of the modal auxiliary “is”. This concurs with the tense of the sentence which wants to know if their sister was given a gift last year. Hence option C is correct.
The modal auxiliary verb “is” indicates third person singular subject, present tense, and is used in sentences which require some form of the auxiliary verb “be”, followed immediately by an -ing verb form. Hence option B is incorrect.
Similarly “are” indicates third person plural subject, whereas the subject is “their sister”, which is singular. Hence option A is incorrect.
The auxiliary verb “does” is substituted for third-person, singular subjects in the present tense. However, as the context refers to the past tense “does” does not concur. Hence option D is incorrect.