What is the primary purpose of using CSS in web design?

The primary purpose of using CSS (Cascading Style Sheets) in web design is to apply styles and layouts. CSS allows web designers to control the visual presentation of HTML content by setting properties such as colors, fonts, spacing, and positioning. This separation of content (HTML) from layout (CSS) not only enhances the aesthetic appeal of a website but also improves maintainability, as changes in the style can be made in one central location without needing to alter the HTML structure.

While creating animations and optimizing images are aspects of web design, they are not the primary function of CSS. Animations can be achieved with CSS, but they represent just one feature among many. Structuring HTML content is the role of HTML itself, which defines the document's structure and meaning. Optimizing images is typically handled through other tools or techniques rather than CSS. Thus, the correct choice highlights the main functionality of CSS in web design.