The correct option is A The reason for hacking the account has not been revealed by the hacker.
The correct answer is ‘The reason for hacking the account has not been revealed by the hacker.’ This is because in the passive voice, the receiver of the action, ‘The reason for hacking the account’ appears first in the sentence and as the receiver is singular, the singular form of the helping verb, ‘has’ is used. It is followed by the negative word ‘not’, the ‘be’ verb form ‘been’, the past participle of the main verb, ‘revealed’, the preposition ‘by’ and the doer of the action ‘the hacker’.