Which file extension indicates that a file is a cascading style sheet?

The file extension that indicates a file is a cascading style sheet is .css. Cascading Style Sheets, commonly referred to as CSS, are crucial in web development as they are used to describe the presentation, layout, and formatting of HTML documents. By linking a .css file to an HTML file, developers can control the visual appearance of web pages, such as colors, fonts, spacing, and overall layout, thus separating content from design. This modular approach allows for more efficient site maintenance and design consistency across multiple pages.

The other options represent different types of files: .html files are used for HTML documents which structure web content, .js files are JavaScript files that add interactivity and dynamic behavior to webpages, and .xml files are used for storing and transporting structured data. Each of these file types serves a unique purpose within web development, but only .css specifically pertains to styling web pages.