Why do we use CSS?

C is the correct choice because CSS, or Cascading Style Sheets, is specifically designed for styling web pages. It allows developers to control the visual presentation of HTML elements, such as colors, fonts, layouts, and spacing. By separating content (HTML) from design (CSS), it enhances the maintainability of websites and improves the overall user experience. This separation means that changes to style can be made in one place, affecting the entire site without altering the underlying markup.

The other options do not align with the primary purpose of CSS. While structuring web pages is the role of HTML, server-side processing typically involves languages like PHP or Python, and managing databases is related to database management systems rather than web styling. Thus, CSS plays a critical role in ensuring that web pages not only function correctly but also look aesthetically pleasing and are user-friendly.