What is the action of “pop” in the context of MPLS switching?
It replaces the top label in the MPLS label stack with a set of labels.

A.
Before the layer 2 header
The label is imposed between the data link layer (Layer 2) header and network layer (Layer 3) header.
The top of the label stack appears first in the packet, and the bottom appears last. The network layer packet immediately follows the last label in the label stack.
B.
Within the Layer 3 header
It replaces the top label in the MPLS label stack with another value.

C.
After the Layer3 header and before Layer2 header
It adds a top label in MPLS label stack.

D.
After the Layer2 header and before Layer3 header
It removes the top label in the MPLS label stack.

Explanation:
Where will the label be imposed in a packet?