Only the students can solve this problem. (Negative)

The correct answer is - ঘ) None but the students can solve this problem.

Affirmative থেকে negative করার ক্ষেত্রে:
- Affirmative-এ subject-এর আগে যদি only থাকে তাহলে তাকে negative করতে হলে none but বসাতে হয়।
- বাক্যের বাকি অংশ একই থাকবে।

Example:
- Aff.: Only the brave deserve the fair.
- Neg.: None but the brave deserve the fair.

More examples:
- Aff.: Only God can help us.
- Neg.: None but God can help us.

- Aff.: Only he can do it.
- Neg.: None but he can do it.

- Aff.: Only an honest man is happy.
- Neg.: None but an honest man is happy.

- Aff.: Only a poet/poetess can write a poem.
- Neg.: None but a poet/poetess can write a poem.

- Aff.: Only a guilty man should be punished.
- Neg.: None but a guilty man should be punished.

Other options
ক) Not the students can solve this problem.
বাক্যটির গঠন ভুল।

খ) None the students can solve this problem.
'but' বাদ পড়েছে, তাই অর্থ পরিবর্তিত হয়েছে।

গ) Only the students cannot solve this problem.
মূল অর্থ উল্টে গেছে (মানে দাঁড়ায় তারা পারবে না)।

Source: A Passage to the English Language by S.M. Zakir Hussain.