Which standard is used for implementing public-key encryption and email message signatures?

The correct choice is determined by recognizing the specific standard that governs public-key encryption and digital signatures for email communications. S/MIME, which stands for Secure/Multipurpose Internet Mail Extensions, is the protocol designed explicitly for securing email messages through encryption and digital signatures. It allows users to send secure emails by enabling the sender to encrypt the message and add a digital signature, ensuring both confidentiality and authenticity.

S/MIME integrates seamlessly into email clients and provides a robust framework for the use of public-key cryptography to achieve secure email communication. In contrast, while PGP (Pretty Good Privacy) also provides similar functionalities for email encryption and signing, it is not a standard in the same formalized capacity as S/MIME, which is widely adopted and endorsed in professional environments.

SSL/TLS and HTTPS are protocols mainly focused on securing web traffic rather than being specific to email encryption or digital signatures. SSL/TLS provides a secure channel over the internet, while HTTPS is merely HTTP running over such a secure channel. They do not deal directly with the complexities of email encryption and signatures the way S/MIME does. Thus, S/MIME is distinctly the recognized standard for the tasks outlined in the question.