Which of the following best describes a CSS selector?

A CSS selector is best described as a pattern used to select the elements you want to style. In CSS (Cascading Style Sheets), selectors determine which HTML elements the defined styles will apply to. They use a variety of identifiers such as tag names, classes, ids, and attributes to target specific elements on a web page. This allows developers to apply consistent styling across multiple elements or to target specific elements for unique styling.

The other options describe concepts that are not relevant to CSS selectors. For instance, a command that runs on the server pertains to server-side programming, which is beyond the scope of CSS. A type of HTML element refers to the building blocks of web content, like <div>, <p>, or <h1>, and does not relate to the function of selectors in styling those elements. Lastly, a tool to optimize images for web pages relates to image processing and not to the styling of HTML elements through CSS. Thus, the essence of a CSS selector lies in its role as a fundamental tool used to apply styling to web page elements.