ACTIVE Voice – Here, the subject PERFORMS the action. There is a simple relationship between the doer of the action and the verb.

Example : She is eating a sandwich.

Passive Voice - Here, the subject receives the action. More importance is given to the action (and not the subject)

Example : A sandwich is being eaten by her.

When we want to change interrogative sentences (questions) from active to passive, we need to be mindful of the FOLLOWING rules:

• The question words how, what,  when, why or where do not change their position when these are used at the BEGINNING of the sentence.

Example :

Active Voice – What were you taking?

Passive Voice – What was being taken by you?

• The verbs has, have, had, will, etc.do not change their position when used at the beginning of the sentence.

Example :

Active Voice – Has she invited Rani?

Passive Voice – Has Rani been invited by her?

• Who changes to by whom and whom changes to who

Example :

Active Voice – Who is eating the cake?

Passive Voice – The cake is being eaten by whom?

Understanding these points, we can now change the given sentence from Active to Passive Voice:

Passive Voice : By whom was the food burned?

Thus, the correct answer is option 3.