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).