Choose the suitable option to complete the sentence.

Many students _______ hard to get first division.

worked

The sentence is in the past tense, so we need a past tense verb. The other options are all incorrect because:

• "Working" is the present participle of the verb "work".
• "Works" is the third-person singular present tense of the verb "work".
• "Had work" is grammatically incorrect.

Therefore, the only correct option is "worked".