What key should Mary use to encrypt a message for Sam so only Sam can read it?

To ensure that only Sam can read the message Mary is sending, she should use Sam's public key for encryption. In asymmetric encryption, which is commonly used in public key cryptography, each participant has a pair of keys: a public key and a private key.

The public key is used to encrypt the message, while the corresponding private key is required to decrypt it. By using Sam's public key, Mary ensures that the message can only be decrypted by Sam using his private key. This method protects the confidentiality of the message, as the public key can be shared openly without compromising security, while the private key must remain confidential to Sam.

Choosing Mary's private key, her public key, or Sam's private key would not achieve the desired outcome of allowing only Sam to read the message. Using Mary's private key would mean only she could read the message after encrypting it, defeating the purpose. Mary's public key would encrypt the message in such a way that only she could access it, and using Sam's private key for encryption would contradict the nature of public key infrastructure, where only the public key is intended for such an operation. Thus, the correct answer, using Sam's public key, ensures that only Sam can decrypt and read the message.