Responsive Web Design is best described as...

Responsive Web Design (RWD) is fundamentally about creating web pages that provide an optimal viewing experience across a wide range of devices, from desktop computers to mobile phones. The key concept is to use fluid grid layouts, flexible images, and CSS media queries to ensure that the design adapts to the size of the viewport.

This approach allows developers to create a single version of a webpage that can scale and adjust to different screen sizes and orientations. By incorporating varying CSS rules, developers can apply specific styles based on the characteristics of the device or screen that is being used to view the website. This eliminates the need to create separate pages for different devices, reducing maintenance overhead and improving the usability of the site.

In contrast, options like focusing solely on mobile design or creating multiple versions of pages for each device type do not embody the core principles of responsive design, which emphasize a unified and adaptable approach. Similarly, designing primarily for desktop and then adjusting for mobile disregards the seamless experience that responsive design seeks to provide.