Which of the following offers confidentiality to an e-mail message?

A.
The sender encrypting it with its private key.

B.
The sender encrypting it with its public key.

C.
The sender encrypting it with the receiver’s public key.

D.
The sender encrypting it with the receiver’s private key.

Explanation:
Public-key encryption, in which a message is encrypted with a recipient’s public key. The message cannot be
decrypted by anyone who does not possess the matching private key, who is thus presumed to be the owner of
that key and the person associated with the public key. This is used in an attempt to ensure confidentiality.
Incorrect Answers:
A: The sender must use the receiver’s key.
B: The sender must use the receiver’s key.
D: The receiver’s public key, not the private key, must be used.

https://en.wikipedia.org/wiki/Public-key_cryptography