What does CSS stand for, and what is its purpose?

Cascading Style Sheets is the correct definition of CSS, and its primary purpose is to style and layout web pages. This includes controlling the visual presentation of HTML elements, such as colors, fonts, spacing, and arrangement on the page. CSS enables designers and developers to create visually appealing and responsive websites, enhancing the user experience by providing the tools to manage how content is displayed across different devices and screen sizes.

The term "cascading" refers to the way styles can be applied in layers, allowing for greater flexibility and control over a webpage's appearance. By using CSS, one can separate content from design, making maintenance easier and improving load times when properly implemented. This separation facilitates a cleaner structure, as developers can manage style changes in one location instead of modifying HTML files directly.

Other answer choices contain misconceptions about CSS’s primary role. For instance, structuring content is typically the domain of HTML, while enhancing web graphics can involve various technologies, including JavaScript and CSS, but is not a direct purpose of CSS alone. Similarly, mobile optimization can be aided by CSS, but it is not its sole purpose, as responsive web design principles integrate a variety of techniques beyond just styling.