NOTE: Official UPSC answer key has considered both B and C as correct.

B. Regret that you cannot be of any help to him. This is a polite and straightforward response that acknowledges your colleague's request while maintaining your professional boundaries. It's a more neutral way of declining the request without going into a detailed explanation, but it still shows respect for your colleague's position.

C. Explain to your colleague that this would be violating the trust of the University authorities and you are not in a position to help him. This option goes a step further by providing a clear rationale for why you cannot help, emphasizing your commitment to ethical standards and the integrity of the examination process.