They should complete the project. (passive)

The correct answer is - ক) The project should be completed by them.

Passive voice of verbs preceded by can, may, should, would etc.
- Verb এর আগে auxiliary verb হিসাবে can, could, should, would, must ইত্যাদি থাকলে শুধু verb অংশের passive গঠন হবে: can/could/should/would/must + be + V(p.p.)

Examples:
- Act: I can do it.
- Pass: It can be done by me.

- Act: You must eat the mango.
- Pass: The mango must be eaten by you.

- Act: He would sing good songs in his boyhood.
- Pass: Good songs would be sung by him in his boyhood.

- Act: She should love her husband.
- Pass: Her husband should be loved by her.

- Act: They may use this tubewell.
- Pass: This tubewell may be used by them.

- Act: Man can eat everything.
- Pass: Everything can be eaten by man.

Other options
খ) The project should completed by them.
বাক্যটি ভুল। এখানে be মিসিং হয়েছে।
Passive voice এর জন্য “should + be + V(p.p.)” গঠন থাকা আবশ্যক।

গ) Should the project be completed by them.
বাক্যটি ভুল। এটি প্রশ্নধর্মী passive sentence (interrogative), statement হিসেবে ব্যবহার করা যায় না।

ঘ) The project completed should by them.
বাক্যটি ভুল। word order ভুল এবং passive voice এর নিয়ম মানা হয় নি।

Source: A Passage To The English Language. S. M. Zakir Hussain.