Final answer:
To confirm Bob sent an email about a client deal, the key aspect to ensure is authenticity of the communication, verifying his identity as the sender.
Step-by-step explanation:
To prove that Bob sent an email regarding a deal with a client, the aspect you would want to know is b) Authenticity. Authenticity ensures that the email was actually sent by Bob and not someone else. While integrity would show that the content of the email has not been tampered with during transmission, timestamp would indicate when the email was sent, and recipient confirmation would ensure that the intended party received the email, it is the authenticity that affirms Bob's identity as the sender.