In Voice change, one can transform the sentence from Active to Passive or Vice versa, but can’t change the sense.
His teacher congratulated him on his brilliant success in the recent examination.

Correct Option: A

In Voice change, one can transform the sentence from Active to Passive or Vice versa, but can’t change the sense. That is, one can’t remove the information conveyed through the sentence or add any additional information. That is the transformation must be done keeping the information intact. That is the reason behind selecting (a) as the answer.
His teacher congratulated him on his brilliant success in the recent examination.

We can say that ,
The above given sentence is given in Passive Voice :- Use structure “auxiliary + past participle”
He was elected Mayor by the people.

Correct Option: B

We can say that ,
The above given sentence is given in Passive Voice :- Use structure “auxiliary + past participle”
He was elected Mayor by the people.
Hence , option B will be correct answer .

We can say that ,
Use structure “auxiliary + past participle”
The above given sentence is given in Passive Voice :-
He was seen picking up a gun by someone.

Correct Option: B

We can say that ,
Use structure “auxiliary + past participle”
The above given sentence is given in Passive Voice :-
He was seen picking up a gun by someone.
Hence ,required answer will be option B .

As per the given above sentence is given in Passive Voice.
Use structure “auxiliary + past participle”
The bell has been rung by the boy.

Correct Option: A

As per the given above sentence is given in Passive Voice.
Use structure “auxiliary + past participle”
The bell has been rung by the boy.
Hence , option A will be correct answer .

We can say that ,
Use structure “auxiliary + past participle”
The above given sentence is given in Passive Voice :-
Your offer cannot be accepted by me.

Correct Option: A

We can say that ,
Use structure “auxiliary + past participle”
The above given sentence is given in Passive Voice :-
Your offer cannot be accepted by me.
Thus, required answer will be option A .