Which key combination would the recipient of a message use to confirm the integrity of the message as well as the authenticity of the sender$?$

a$.$ The sender$\text{'}$s public key and The recipient's private key

b$.$ The sender$\text{'}$s public key and the recipients public key

c$.$ The sender$\text{'}$s private key and the recipient's public key

d$.$ The sender$\text{'}$s private key and the recipients private key