Which layer defines the X.25, V.35, X,21 and HSSI standard interfaces?

A.
Transport layer

B.
Network layer

C.
Data link layer

D.
Physical layer

Explanation:
The Physical Layer is the layer that is concerned with the signaling of the message and the interface
between the sender or receiver and the medium. The physical layer is generally defined by one of
the standards bodies and carries a designation that indicates the characteristics of the connection.
Among frequently used physical layers standards are EIA-232-D, ITU V.35, and some of the X series
(X.21/X.21bis, for example).