To whom does a PE-router advertise labels, when operating in the default frame mode?

A.
The PE-router advertises a label to all of its neighbors.

B.
The PE-router only advertises labels to its neighbors in the MPLS domain.

C.
The PE-router only advertises labels to its neighbors outside the MPLS domain.

D.
The PE-router does not advertise labels.
Label advertisement is only done by the core LSR.

Explanation:

The Ingress Edge-LSR receives an IP packet, classifies the packet into a forward equivalence class (FEC), and labels the packet with the outgoing label stack corresponding to the FEC. For unicast destination-based IP routing, the FEC corresponds to a destination subnet and the packet classification is a traditional layer 3 lookup in the forwarding table.
Reference: MPLS and VPN Architectures (Ciscopress) page 25