Change the following from active to passive: The agent could have sold the house.

Correct — A. The verb is could have sold: a modal, then have + past participle.

To make it passive, keep the modal and have, add been, and keep the past participle:
could have sold → could have been sold

The object the house becomes the subject, and the doer moves into a by-phrase. The house could have been sold by the agent → option (a).

Why the others are wrong

• (b)
  The house could be sold by the agent. — could be sold is the passive of could sell, not of could have sold. Dropping have moves the possibility from the past into the present.
• (c)
  The house could have sold by the agent. — been is missing. A passive needs a form of be before the past participle, and could have sold leaves the active verb unchanged.
• (d)
  The house could had been sold by the agent. — could had breaks the modal rule. A modal is followed by the bare infinitive, so after could the form is have, never had.

The passive is be + past participle, and the form of be copies the tense of the active verb.

After a modal perfect, be takes the form been: modal + have been + past participle.

must have written → must have been written
should have told → should have been told
could have sold → could have been sold

Key facts

• Passive of a modal perfect: modal + have been + past participle.
• A modal is followed by the bare infinitive: could have, never could had.
• Could be sold is the passive of could sell.
• In the passive the active object becomes the subject, and the active subject moves into a by-phrase.

Common traps

• Choosing (c) because it keeps every word of the original verb, when the passive needs been
• Choosing (b) because it reads more simply, when dropping have changes the time
• Writing could had been by letting the past tense spill onto have

No directly related past PYQ was found.