Which layer of the OSI model does the problem most likely exist?

You enabled CDP on two Cisco Routers which are connected to each other. The Line and Protocol status for the interfaces on both routers show as UP but the routers do not see each other a CDP neighbors. Which layer of the OSI model does the problem most likely exist?

A.
Physical

B.
Session

C.
Application

D.
Data-Link

E.
Network