How do I perform encryption?

a) You can encrypt messages using private-key encryption. In private-key encryption, the two parties involved in sending and receiving the message, as well as the server through which the e-mail is sent have the code.
b) You can encrypt messages using private-key encryption. In private-key encryption, the sender encrypts a message using the receiver's private key, which is placed on a Web site, for instance.
c) You can encrypt messages using private-key encryption. In private-key encryption, only the two parties involved in sending and receiving the message have the code.
d) You can encrypt messages using public-key encryption. In public-key encryption, anyone receiving the message can decode with the key sent in the e-mail.