Which one of the following is the Open Systems Interconnection (OSI) protocol for message
handling?

A.
X.25

B.
X.400

C.
X.500

D.
X.509

Explanation:
An ISO and ITU standard for addressing and transporting e-mail messages. It
conforms to layer 7 of the OSI model and supports several types of transport mechanisms,
including Ethernet, X.25, TCP/IP, and dial-up lines. -http://www.webopedia.com/TERM/X/X_400.html