Answer B (AWS Global Accelerator) best meets both requirements: fastest failover from application failures and the lowest response time for global users. Global Accelerator is designed for global, latency-sensitive, highly available applications. It uses static Anycast IP addresses so users enter the AWS network at the nearest edge location, then traffic is carried over the AWS global backbone to the closest healthy regional endpoint (in this case, each Region’s ALB). It continuously performs health checks and can rapidly shift traffic away from unhealthy endpoints, which helps “handle application failures quickly” without being constrained by DNS caching behavior.

Why the other options are less suitable:

C (Route 53 latency-based routing + health checks) can improve routing and failover, but it is still DNS-based. DNS-based failover can be delayed by DNS resolver caching and TTL propagation, meaning failover may not be as fast or as consistent as Global Accelerator for rapid recovery.

A (CloudFront with origin group) can provide origin failover and performance benefits mainly for cacheable or edge-optimizable content. For a payment processing API (typically dynamic and not cache-friendly), CloudFront is not the primary service for achieving the least response time globally with fast multi-Region failover compared with Global Accelerator’s Anycast + edge routing model.

D (AWS Cloud WAN) addresses private network connectivity and centralized network policy across Regions/VPCs. It does not directly optimize public user-to-application latency or provide the same kind of edge-based, Anycast-driven rapid failover for internet-facing API endpoints.