Convert the sentence provided below from its passive voice structure to an active voice structure: The documents had been submitted by the assistant before the deadline.

Correct — B. The passive verb is had been submitted: past perfect had, plus the passive been + past participle.

To make it active, drop the passive been and keep the tense:
had been submitted → had submitted

The agent the assistant becomes the subject, the documents become the object, and before the deadline stays put. That is option (b).

Why the others are wrong

• (a)
  The assistant submits the documents before the deadline. — submits is simple present. Its passive would be the documents are submitted, so the past perfect of the original is lost.
• (c)
  The assistant was submitting the documents before the deadline. — was submitting is past continuous. Turned passive it would give were being submitted, not had been submitted.
• (d)
  The assistant is submitting the documents before the deadline. — is submitting is present continuous, whose passive is are being submitted. Both the time and the aspect of the original change.

Voice changes, tense does not. Going from passive to active swaps the roles: the by-phrase becomes the subject and the passive subject becomes the object. The tense must survive the change.

Map the verb by removing the passive be:
is submitted → submits
was being submitted → was submitting
had been submitted → had submitted

Key facts

• Past perfect passive: had been + past participle. Its active form: had + past participle.
• The agent in the by-phrase becomes the subject of the active sentence.
• Adverbials such as before the deadline carry over unchanged.

Common traps

• Picking a present form (submits, is submitting) because it sounds more natural than the past perfect.
• Keeping been in the active, which gives had been submitting, a different tense.

No directly related past PYQ was found.