Carl is a security professional in the health care industry. He needs to ensure that company emails that contain patient health information are encrypted. Which of the following is the best solution?

a$.$

Diffie$-$Hellman encryption

b$.$

Secure$/$Multipurpose Internet Mail Extensions $($S$/$MIME$)$

c$.$

X$\mathrm{.}509$ standard

d$.$

Payment Card Industry Data Security Standard $($PCI DSS$)$