The supplicant creates a message digest by adding the password to the challenge message and hashing the two hashing the plaintext message encrypting the message digest with its own private key none of the above.

The supplicant creates a message digest by hashing the plaintext message.
The verb to hash basically means to chop, meaning that you make something large into a smaller version. So, if you want to digest a message, meaning to shorten it, to summarize it, you will have to hash, or shorten the plaintext message.

The correct answer is d