This answer is correct because it follows the best practice design for routing the wireless traffic in a VHD wireless solution. By having the MCs act at Layer2, the architectcan enable the use of clustering, which provides high availability, load balancing, and seamless roaming for wireless users. By having the core routing switches act as the default gateway, the architect can avoid tunneling the wireless traffic to the MCs, which reduces the network overhead and latency. This also allows the core routing switches to apply security policies and QoS to the wireless traffic based on the user roles assigned by the MCs. References:
Aruba Certified Design Professional Official Certification Study Guide (HPE6-A47) - Chapter 6) Aruba Validated Reference Design: Very High Density 802.11ac Networks - Chapter 4) Aruba Mobility Controller Clustering - Data sheet)