Since each of the participiants has a key pair in which one of the keys is public and the other one is private, the communication between participiants are provided with public key cryptography.  Data encrypted by the private key can only be decrypted by its public key counterpart. Data encrypted with a public key can only be decrypted with its private key. If John encrypts a message with his private key both July and Mary can decrypt the message. Because they know John's public key. If John encypts a message with Mary's public key, only Mary can decrypt the message. If John takes a message encrypted by his public key, the message may be sent by either Mary or July because both of them know John's public key. Correct answer is A.