What is the name of the command used by SMTP to transmit email over TLS?

Email is transmitted across the Internet using the Simple Mail Transport Protocol. SMTP does not encrypt email, leaving the information in the message vulnerable to being read by an unauthorized person. SMTP can upgrade a connection between two mail servers to use TLS. Email transmitted by SMTP over TLS is encrypted.

What is the name of the command used by SMTP to transmit email over TLS?
A . OPPORTUNISTICTLS STARTTLS
B . FORCETLS
C . UPGRADETLS

Answer: B