Answer: A

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
his/her 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).