Select the option that expresses the given sentence in active voice. By whom was grammar taught to you?

Correct — B. The passive verb is was taught, a simple past, and the agent is the question word by whom.

In the active, that agent becomes the subject: by whom turns into Who, and the verb sheds the be-form to leave taught — same tense, no be, no by.

Grammar, the passive subject, returns as the object, and to you drops its preposition: Who taught you grammar?, option (b).

Why the others are wrong

• (a)
  You were taught grammar by whom? — You were taught grammar by whom? keeps were taught and the by-agent, so it is still passive. Only the word order has moved, and reordering a passive does not make it active.
• (c)
  Who had taught you grammar? — Who had taught you grammar? is active, but it shifts the tense to past perfect. A voice change keeps the tense it was given, and the original is the simple past was taught.
• (d)
  Did he teach you grammar? — Did he teach you grammar? swaps the question word whom for a definite he, so it asks whether one named person did it instead of asking who the agent was.

Changing the voice of a question follows the same three steps as a statement, with one extra: the question word has to keep its job.

By whom is the agent, so in the active it becomes the subject who. The passive subject moves into object position, and the be-form disappears.

Tense is the one thing that must not move. was taught is simple past, so the active is taught — not had taught, not has taught.

Key facts

• Passive to active: the by-agent becomes the subject, the passive subject becomes the object, and the be-form is dropped.
• A voice change never changes the tense, so was taught becomes taught, not had taught.
• When the agent is the thing being asked about, by whom becomes the subject who.
• In Who taught you grammar?, you is the indirect object and grammar the direct object, recovering the passive to you.

Common traps

• Reordering the passive into You were taught grammar by whom? and calling that the active.
• Upgrading the tense to had taught because the sentence feels like an enquiry into the past.
• Turning the wh-question into a yes/no question, which changes what is being asked.

No directly related past PYQ was found.