how are the second (VPN) labels in the label stack propagated from the egress PE router to the ingress PE router?

In MPLS VPN implementations, how are the second (VPN) labels in the label stack propagated from the egress PE router to the ingress PE router?

A.
The core IGP.

B.
MP-IBGP VPNv4 routing updates.

C.
MP-IBGP IPv4 routing updates.

D.
EBGP IPv4 routing ipdates

E.
LDP

F.
TDP