The correct answer is option 4.As previously indicated, X adds a digital signature to message M, encrypts M, and sends it to Y.The following key sequences are used for the operations to ensure that Y receives the message and is able to decrypt it.Encryption Xs private key followed by Ys public key In order to guarantee authenticity and confidentiality, the message should be encrypted using the sender's private key and receiver's public key.Decryption Ys private key followed by Xs public key Receiver Y will need to decrypt it using its own private key and send X's public key when received with the encrypted message.Hence the correct answer is Encryption: Xs private key followed by Ys public key; Decryption: Ys private key followed by Xs public key.