What do you want to ""know"" to prove yourself that it was Bob who had send a mail?

Bob, your senior colleague, has sent you a mail regarding a deal with one of the clients. You are requested to accept the offer and you oblige. After 2 days. Bob denies that he had ever sent a mail.

What do you want to ""know"" to prove yourself that it was Bob who had send a mail?
A . Authentication
B . Confidentiality
C . Integrity
D . Non-Repudiation

Answer: D