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• The correct passive voice is: গ) The project has been completed by them.

• Active voice থেকে passive voice করার নিয়ম:
- Active voice এর object টি passive voice এর subject হয়।
- Tense অনুযায়ী auxiliary verb বসে।
- তারপর মূল verb এর past participle হয়।
- preposition (by, with, at, to, in) বসে।
- Active voice এর subject টি passive voice এর object হয়।

Present perfect tense এর passive voice করার নিয়ম:
- Object টির subjective form + have been/ has been + মূল verb এর past participle + by + subject এর objective form.

সেই অনুসারে,
Active voice: They have completed the project.
Passive voice: The project has been completed by them.

Other Examples:
1.
Active: He has written a letter.
Passive: A letter has been written by him.

2.
Active: They have finished the work.
Passive: The work has been finished by them.