The correct option is C were not answered
A sentence in passive voice focuses on the person or thing that receives
an action rather than the person/thing that performs the action.
For example, 'My room was cleaned by me.'
The basic structure of passive voice is:
the appropriate form of the verb 'to be' + past participle
Option D: 'Were not answered' is
the appropriate structure, formed by the past tense verb 'were'
+ not + past participle (answered). Hence option D is correct.
Option A: 'Was' is a singular verb whereas the subject (questions) is plural. The
subject and verb must agree with one another in terms of number. Hence A
is incorrect.
Option B: 'Did' is a verb in the active voice. The word itself means 'to perform an action'. It changes the subject to perform, rather than receive, an action. Hence B is incorrect.
Option C: 'Answer' is the base verb that does not express the past tense of the sentence. The past participle 'answered' must be used. Hence C is incorrect.