What is a CSS preprocessor?

A CSS preprocessor is indeed a tool that extends CSS with additional features. It enhances the capabilities of standard CSS by allowing developers to use variables, nested rules, mixins, functions, and other programming constructs that simplify the process of writing stylesheets. By incorporating these features, developers can create more maintainable and efficient CSS code, which is particularly beneficial for larger projects where organization and reuse of styles are important.

Options that suggest optimizing images, modifying HTML structure, or compressing CSS files do not accurately describe the function of a CSS preprocessor. Each of these tasks pertains to different aspects of web development, but a CSS preprocessor specifically addresses the limitations of traditional CSS and aims to streamline the styling process.