Choice "D" is correct. Decryption or decipherment is the step where the intended recipient converts the cipher text into plain text.
Choice "c" is incorrect. Encryption involves using a password or a digital key to scramble a readable (plain text) message into an unreadable (cipher text) message.
Choice "a" is incorrect. Digital certificates are yet another form of data security. It behaves in the online world the same way driver's licenses, passports, and other trusted documents behave outside the online world.
Choice "b" is incorrect. The term public key infrastructure (PKI) refers to the system and processes used to issue and manage asymmetric keys and digital certificates.