The LDP identifier is used to identify a router during LDP session establishment.
What size is the LDP identifier?

A.
8 bytes

B.
6 bytes

C.
2 bytes

D.
4 bytes

Explanation:

From page 33 of reference "The TDP identifier is determined in the same way as the OSPF or BGP identifier (unless controlled by the tag tdp router-id command) — the highest IP address of all loopback interfaces is used. That means the identified is a 32-bit ip address which should be 32/8 = 4 bytes long.
Reference: MPLS and VPN Architectures (CCIP edition)