E-mail message authenticity and confidentiality is BEST achieved by signing the message using the:

A. senders private key and encrypting the message using the receivers public key.

B. senders public key and encrypting the message using the receivers private key.

C. receivers private key and encrypting the message using the senders public key.

D. receivers public key and encrypting the message using the senders private key.

Explanation:

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