Choose the correct passive form from the options given below:

A cardigan ___________ by the old lady at that time.

a) Was knitted
b) Has knitted
c) Was being knitted
d) Was knit

Hint: In a sentence, an action can be represented in either active or passive voice.

Complete answer:

We have to find out the correct passive form in this sentence. The passive voice in any sentence always uses the past participle form of the main verb, whichever tense it may be.
The sentence in the question is in the active voice. In a sentence, only the auxiliary verbs depend on the active voice.

Let us analyze the options given to us in this question:
Option (a.), 'Was knitted', refers to simple past tense.
Therefore, option (a.) is incorrect as 'at that time’ refers to past continuous tense.

Option (b.), ‘has knitted', is in the present perfect tense.
Therefore, option (b.) is incorrect as 'at that time’ refers to past continuous tense.

Option (c.), ‘was being knitted', is in the past continuous tense.
Therefore, option (c.) is correct as 'at that time’ refers to past continuous tense.

Option (d.), ‘was knit', is incorrect as ‘knit’ is not used in its third form.

Note: In all active/passive voice questions, you should gauge whether the action is happening in the past, present, or future first. Then keeping that in mind, use the right tense of the action in the sentence.