Final answer:
Both Nginx and HAProxy are capable of handling millions of requests per second, with Nginx known for its high performance and HAProxy for high availability and proxying capabilities.
Step-by-step explanation:
The question asks which load balancer can handle millions of requests per second. Among the listed options — Nginx, Apache, HAProxy, and IIS — Nginx and HAProxy are both capable of handling such a high load, given proper configuration and sufficient underlying hardware resources. Nginx is known for its high performance and scalability, often used in serving static content and as a reverse proxy. HAProxy, meanwhile, is a free, open-source software that offers high availability, load balancing, and proxying for TCP and HTTP-based applications. While Apache and IIS can also handle a significant number of requests, they are generally not as performant as Nginx or HAProxy when scaling to millions of requests per second.