The correct answer is C — Secure/Multipurpose Internet Mail Extensions (S/MIME).

As outlined in WGU Cybersecurity Architecture and Engineering (KFO1 / D488), S/MIME provides encryption, integrity, and authentication for email messages by using public key cryptography and digital signatures.

SMTP (A) is used for sending emails but does not secure them. PGP (B) also secures emails but is not a protocol; it is a program and standard. IPsec (D) secures network communications, not email specifically.

Reference Extract from Study Guide:

"Secure/Multipurpose Internet Mail Extensions (S/MIME) is a protocol used to encrypt and digitally sign email communications, ensuring confidentiality, integrity, and authenticity."

— WGU Cybersecurity Architecture and Engineering (KFO1 / D488), Secure Communication Technologies

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