The correct option is C was given
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 C has the most appropriate form: the
simple past form of 'to be', 'was'.
'Was given' is
the required structure, formed by the past tense verb 'was'
+ past participle (given). Hence option C is correct.
Option A: 'Were' is a plural verb
whereas the subject 'A treat' is singular. The subject and verb must agree
with one another in terms of number. Hence A is incorrect.
Option B:
'Gave' is the simple past tense form of the verb 'to give'. It is in
the active voice whereas the required form must be in the passive voice.
The verb 'gave' changes the subject to perform, rather than receive,
an action. Hence B is incorrect.
Option D: 'Was gave' is grammatically incorrect, as 'gave' is the simple past tense form of the verb. The correct form of the verb is the past participle, given. Hence D is incorrect.