What is a CSS preprocessor?

A CSS preprocessor is indeed a scripting language that enhances the capabilities of standard CSS by introducing features such as variables, nested rules, and functions. This allows developers to write more maintainable and dynamic stylesheets. For example, with variables, a developer can store color values or other commonly used styles and refer to them throughout the stylesheet, making it easier to manage and update styles globally. Nested rules enable a more structured hierarchy in styles, reflecting the HTML structure better, which can lead to cleaner and more understandable code. Functions can provide calculations and operations on values, adding further flexibility.

The other options represent different tools or concepts that do not accurately define a CSS preprocessor. While tools for compressing CSS files or libraries for responsive design enhance CSS functionality in their own way, they do not extend the syntax or abilities of CSS like preprocessors do. Similarly, a framework for organizing JavaScript and CSS together addresses the structuring and usability of code but is separate from the enhancements offered by preprocessors.