E-mail message authenticity and confidentiality is BEST achieved by signing the message using the:
( A) sender’s private key and encrypting the message using the receiver’s public key.

A.
sender’s public key and encrypting the message using the receiver’s private key.

B.
receiver’s private key and encrypting the message using the sender’s public key.

C.
receiver’s public key and encrypting the message using the sender’s private key.

Explanation:
By signing the message with the sender’s private key, the receiver can verify its authenticity using the sender’s public key. By encrypting the message with the receiver’s public key, only the receiver can decrypt the message using their own private key. The receiver’s private key is confidential and, therefore, unknown to the sender. Messages encrypted using the sender’s private key can be read by anyone with the sender’s public key.