If this sentence is changed to active voice, then the subject is also changed, in order to retain the meaning of the sentence.

So, the object takes the place of the subject and vice versa. The verb form of the sentence is also changed from passive form to active.

'Subject (He) + verb (was obliged) + infinitive (to resign)' changes to:

'New subject (Circumstances) + verb (obliged) + new object (him) + infinitive (to resign)'

The active form of 'was obliged' in the simple past tense is 'obliged '. Since the passive form does not have an object, the object is implied in the sentence: He was obliged to resign by circumstances, where 'circumstances' is assumed to be the object.

This can be seen in option C, hence it's the right answer.

Option A: The verb form 'was make to' is wrong since 'make' is not a past tense verb.

Option B: The sentence has been changed in structure. The verb 'obliged' has been changed to the noun 'obligation'.

Option D: Similarly, 'resign' is a verb form that has been changed to the noun 'resignation'. Also, the meaning of the sentence has changed.