What best describes a CSS sprite?

A CSS sprite is best described as a single image file containing multiple images. This technique involves combining several images into one larger image file, which helps reduce the number of server requests needed when loading a webpage. Instead of loading multiple individual images, a browser can load one sprite image and then use CSS to display only the part of the image that corresponds to each specific element on the webpage. This optimization can significantly improve site performance, making it quicker and more efficient by lowering load times.

The other options do not accurately capture what a CSS sprite is. For instance, animations created using CSS refer to a different aspect of styling, focusing on transforming visual elements over time rather than combining image resources. Responsive design relates to adjusting layout and visuals based on screen size and resolution, which also does not pertain to the concept of CSS sprites. Lastly, an external stylesheet format describes the organization of CSS code rather than a graphical technique, further distancing it from the definition of a sprite.