Which function does LDP provide in an MPLS topology?

Which function does LDP provide in an MPLS topology?
A. It enables a MPLS topology to connect multiple VPNs to P routers.
B. It provides hop-by-hop forwarding in an MPLS topology for LSRs.
C. It exchanges routes for MPLS VPNs across different VRFs.
D. It provides a means for LSRs to exchange IP routes.