Bob, your senior colleague, has sent you a mail regarding a deal with one of...

Comprehensive and Detailed Explanation:

Non-repudiation ensures that a sender cannot deny having sent a message. This is typically achieved through digital signatures or logs which verify the origin and integrity of communications.

From CEH v13 Courseware:

Module 10: Cryptography → Security Services

“Non-repudiation prevents entities from denying their actions, such as sending emails or digital transactions.”