The correct answer is A: Implement TLS on the NLB with a server certificate. This is because TLS (Transport Layer Security) ensures that data in transit is encrypted between the client and the load balancer, providing a secure channel to protect data from potential eavesdropping and tampering. Configuring a TLS listener on the NLB and deploying a valid server certificate addresses the requirement for improving the security of data in transit effectively. Other options mentioned either do not address the data-in-transit security directly or are related to different aspects of security like shielding against attacks or at-rest encryption.