Which line from the output of the show ip interface command indicates that there is a Layer 1
problem?

A.
Serial0/1 is up, line protocol is up

B.
Serial0/1 is down, line protocol is down

C.
Serial0/1 is administratively down, line protocol is down

D.
Serial0/1 is up, line protocol is down

Explanation:
When the physical interface itself is down, then the problem is related to layer 1. When it is up,
but the line protocol is down, then the problem is related to layer 2.

http://www.cisco.com/univercd/cc/td/doc/cisintwk/itg_v1/tr1915.htm