The controller has a global view of the network, and it can easily ensure that...

A (Scaling challenges for services):Centralized SDN controllers may experience scalability challenges for distributed services like DHCP and load balancing since these often require real-time responsiveness and distributed service locality.

D (Latency in reactive handling):Centralized controllers may introduce additional latency during reactive PACKET_IN event handling as control decisions traverse the control plane before forwarding happens.

Other options explained:

B: Centralized controllers still present failure domain risks; high-availability must be explicitly designed.

C: Smart NICs operate primarily at the host level, often independently of centralized SDN controllers.

E: Legacy equipment often lacks support for many SDN southbound APIs.