The correct option is C was chosen
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 of the verb: the
simple past form of 'to be', 'was'.
'Was chosen' is
the required structure, formed by the past tense verb 'was'
+ past participle (chosen). Hence option C is correct.
Option A: 'Chose' is the simple past tense form of the verb 'to choose'. It is in the active voice whereas the required form must be in the passive voice. The verb 'chose' changes the subject to perform, rather than receive, an action. Hence A is incorrect.
Option B: 'Chosen' is a past
participle which requires a helping verb (auxiliary: was) to help
determine the tense. Without this helping verb, the sentence sounds
incomplete. Hence B is incorrect.
Option D: 'Were' is a plural verb whereas the subject Mallika is singular. The subject and verb must agree with one another in terms of number. Hence D is incorrect.