Which of the following describes CSS?

The correct choice defines CSS as a style sheet language specifically designed for controlling the layout and appearance of web pages. CSS, or Cascading Style Sheets, allows developers to separate content from design, offering a streamlined approach to styling HTML documents. This means that developers can define visual styles for elements, including colors, fonts, spacing, and overall layout, without altering the HTML structure itself.

This separation enhances maintainability and consistency across web pages since changes to style can easily be made in one location, affecting all linked pages. Additionally, CSS is crucial in responsive web design, enabling sites to adapt visually to different screen sizes and devices.

The other options suggest concepts that do not align with CSS's purpose. For instance, a programming language for backend development typically involves languages such as Python, Java, or PHP, which are focused on server-side logic and database interactions. A markup language for defining document structure refers to languages like HTML, which outline the elements of web content but do not deal with styling. Lastly, a data format for web application settings could refer to formats like JSON or XML, which are used to structure data rather than style it.