In a two-tier campus design, the core and distribution functions are collapsed into one layer. That collapsed core provides aggregation, Layer 3 boundaries, routing between access-layer blocks, and policy enforcement that would otherwise sit at the distribution layer. The access layer attaches users and endpoints; it is not the collapsed core ' s role. Marking interesting traffic can occur as part of QoS policy, but it is not the defining architectural function. Applying security policies may happen in some designs, but the answer that best describes collapsed-core responsibility is enforcing routing policies. Cisco CCNA 200-301 v1.1 Network Fundamentals expects candidates to compare two-tier and three-tier architectures. In small and medium campus networks, a collapsed core reduces complexity by combining distribution and core into the same devices. The key function in the answer set is routing-policy enforcement. Therefore, A is the best answer.