Which of the following is a popular web server software?

The correct choice is "All of the above" because Nginx, Apache HTTP Server, and Internet Information Services (IIS) are all widely recognized and utilized web server software. Each of these servers has its own unique features and strengths:

Nginx is known for its high performance, scalability, and efficient handling of multiple connections. It is often used as a reverse proxy and load balancer for serving static content and for application delivery.

Apache HTTP Server has been one of the most popular web servers for many years, notable for its flexibility, strong community support, and rich feature set. It is highly customizable, thanks to its modular architecture.

IIS, developed by Microsoft, is designed primarily for Windows environments. It integrates well with other Microsoft products and provides a user-friendly interface for hosting websites and applications on Windows servers.

Attributing popularity to these servers involves considering their widespread deployment in various environments, their active user communities, and the extensive documentation available for users, making them significant players in the web server landscape. Therefore, the choice of "All of the above" captures the importance of each of these options in the realm of web server technologies.