A teacher should refuse a request to give a student an unfair examination advantage.
Why C fits: A polite but firm refusal protects fairness and professional integrity.
Why the others are weaker: Granting the favour is unethical; evasion leaves the request unresolved; a rude reply is unnecessary.
In practice: The teacher can explain that assessment rules apply equally to all students and, if pressure continues, report the request through the institution’s appropriate procedure.