All IP addresses within a 10.0.0.0/16 VPC are fully utilized with application servers across two Availability Zones. The application servers need to send frequent UDP probes to a single central authentication server on the Internet to confirm that it is running up-to-date packages. The network is designed for application servers to use a single NAT gateway for internal access. Testing reveals that a few of the servers are unable to communicate with the authentication server.
What is the reason for this failure?

A. The NAT gateway does not support UDP traffic.
B. The authentication server is not accepting traffic.
C. The NAT gateway cannot allocate more ports.
D. The NAT gateway is launched in a private subnet.