When Receiving a Message from Somebody Else and Decrypting It

Multiple Choice

When receiving a message from somebody else and decrypting it using the RSA encryption algorithm, what key do we use to decrypt the message?

A) None; the message was not encrypted.
B) None; we do not use a key to decrypt our message.
C) We use the message sender's private key.
D) We use the message's sender public key.