Change the following from active to passive: Where will they hold the conference?

Correct — B. The active verb is will hold, and its object is the conference.

In the passive the object becomes the subject, and will hold becomes will be held. They names nobody in particular, so by them is dropped.

A direct question keeps question order: Where + will + subject + be held. Where will the conference be held? → option (b).

Why the others are wrong

• (a)
  Where the conference will be held? — Where the conference will be held is statement order. It belongs inside a longer sentence, as in Tell me where the conference will be held, not in a direct question.
• (c)
  Where is the conference held? — is held is the present simple passive, so it turns will hold into a present tense. The passive keeps the tense of the active, and will hold is future.
• (d)
  Where has the conference been held? — has been held is the present perfect passive and speaks of what has already happened. The active will hold is future.

The passive is be + past participle, and be takes the tense of the active verb:
will hold → will be held
is holding → is being held
held → was held

In a question the first auxiliary goes before the subject. With will be held, that auxiliary is will: Where will the conference be held?

Key facts

• Passive of will + verb: will be + past participle, as in will be held.
• In a direct wh-question the auxiliary comes before the subject: Where will the conference be held?
• A vague doer such as they, people or someone is normally left out of the passive: They hold the fair every year → The fair is held every year.
• Statement order after a wh-word belongs to indirect questions: Tell me where it will be held.

Common traps

• Choosing statement order because it reads smoothly, when a direct question needs will before the subject
• Letting the tense slip: will hold must stay future, which rules out is held and has been held

No directly related past PYQ was found.