The sentence that is most relevant to the argument that Zimmermann's PGP was easy to use is "To send a message to Bob, Alice would simply write her e-mail and select the PGP option from a menu on her computer screen".
The element that most clearly indicates that this sentence supports the argument that Zimmermann's PGP was easy to use is the word "simply", which is used to refer to something that can be done in an easy way since it does not require to follow many steps. PGP stands for Pretty Good Privacy, an encryption program mainly used for encrypting or decrypting different types of files. PGP was created by Zimmerman in 1991.