A sentence has been given in Active/Passive Voice. Out of the four given…

Explanation:

Correct passive form: By whom can Nehru’s integrity be questioned?

How to convert the active question into passive:

• Step 1: Identify subject and object in the active question. Active: "Who can question Nehru’s integrity?" — agent = who, object = Nehru’s integrity.

• Step 2: Make the object the subject of the passive: "Nehru’s integrity".

• Step 3: Use the modal + passive structure: modal 'can' + 'be' + past participle 'questioned' → "can be questioned".

• Step 4: Express the agent with a 'by' phrase. For a formal wh-question start with 'By whom' and invert the auxiliary and subject: "By whom can Nehru’s integrity be questioned?"

Why the other choices are not correct or not preferred:

• The sentence "By whom Nehru’s integrity can be questioned ?" uses incorrect auxiliary-subject order for a question; 'can' must come before the subject in a wh-question.

• The sentence "Nehru’s integrity can be questioned by whom ?" is grammatically understandable but sounds informal or surprised because it keeps declarative word order and places 'by whom' at the end; the formal passive question uses inversion at the start.

• The sentence "Who could have questioned Nehru’s integrity?" changes the modality and time (introduces 'could have'), so it does not preserve the original present-tense meaning.