چار آپشن میں سے کسی ایک پر کلک کرنے سے جواب موٹا ہو جاے گا
While changing an interrogative sentence to passive voice, the subject 'who' changes to an object 'by whom'.
The passive form of 'taught' in the simple past tense is 'was/were taught.
This can be seen in option C, hence it's the right answer.
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ND4-12-2022
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