The correct option is A You and he were seen by us.
A sentence is in active voice when the subject is the doer of the action.
A sentence is in passive voice when the new subject (object in active voice) is the receiver of that action.
The object in the active voice now takes the place of the subject, and vice versa. (subject + verb + object, where the subject and object swap places)
In the given sentence, the subject (We) performs the action(saw) towards the object (you and him). The object here is compound, as it contains two people, you and him.
Also, the sentence is in simple past tense, hence the tense will change in terms of passive voice.
Option B: Here the object receives the action. 'Were seen' is in simple past tense in passive voice.
Also, 'him' changes to 'he' as the former is an object pronoun that changes to subject pronoun as the voice changes. Hence B is the correct option.
Option A : 'Him' has not changed here, although it must be changed to 'he'. Also, the verb 'was' is singular whereas the new subject (you and he) is plural. Hence A is incorrect.
Option C: Similarly, 'him' is incorrect in C.
Option D: The auxiliary verb 'were' is missing here. Since it's in passive voice, 'were' is required to indicate the simple past tense. Without it, the sentence sounds incomplete. Hence D is incorrect.