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)

According to the Designing Aruba Solutions course, dedicated Air Monitors (AMs) are APs that are configured to scan all channels for wireless threats and perform wireless intrusion prevention system (WIPS) functions. One of the advantages of using dedicated AMs over APs that are doing WIPS in AP mode (hybrid) is that AMs can implement wireless containment on any channel on which they detect a threat without affecting the service of legitimate clients. Wireless containment is a technique that prevents rogue APs or clients from communicating with the network by sending deauthentication or disassociation frames to them.
However, if an AP that is serving clients performs wireless containment on a different channel, it will have to switch channels frequently, which will degrade the performance and user experience of the clients. Therefore, dedicated AMs are recommended for customers who need effective and efficient wireless containment without compromising client service.
References:
Designing Aruba Solutions, Rev. 20.11, Module 5: Aruba Security Design, page 5-13 Aruba Validated Reference Design: Wireless Intrusion Prevention System, page 10

A wide sector directional antenna is a type of antenna that radiates or receives radio waves in a sector-shaped pattern, covering a wide angle of a circle. The width of the sector is determined by the horizontal beamwidth (H-plane) of the antenna, while the height of the sector is determined by the vertical beamwidth (E-plane) of the antenna12. A wide sector directional antenna is usually used to provide regional coverage for wireless networks, such as Wireless Internet Service Providers (WISP) or similar applications2.
According to the antenna specifications given, Antenna 3 has the widest horizontal beamwidth of 100 degrees, which means it can cover a large sector of a circle. Antenna 4 has a narrower horizontal beamwidth of 60 degrees, which means it can cover a smaller sector of a circle. Antenna 1 and Antenna 2 have a horizontal beamwidth of 360 degrees, which means they are omnidirectional antennas, not directional antennas.
Therefore, Antenna 3 is the best choice for the plan that specifies wide sector directional antennas.
References:
1: https://en.wikipedia.org/wiki/Sector_antenna
2: https://lcantennas.com/directional-antennas/