Company XYZ is designing their network using the three-layer hierarchical model.

A (Access):QoS classification and marking should occur as close to the traffic source as possible—at the access layer—so that policies can be consistently applied throughout the network.

Other options explained:

B/D: QoS policies at these layers rely on markings applied earlier.

C: Collapsed core is simply core and distribution merged, but marking should still happen at access.