What character is used to denote an ID selector in CSS?

In CSS, the pound sign (#) is used to denote an ID selector. An ID selector is a powerful way to style a single, unique element on a webpage. When you use a pound sign, it signals that the styles that follow are to be applied specifically to the HTML element with that particular ID attribute.

For example, if you have an HTML element like <div id="header">, you would write the CSS rule as #header { /* styles here */ }. This tells the browser to apply the styles contained within the curly braces only to the element with the ID of "header."

The other characters mentioned do not serve as ID selectors in CSS. The period is used for class selectors, the hyphen can be part of naming conventions but does not signify any type of selector, and the at symbol is used in CSS for at-rules like @media or @import, but not for selecting elements based on their ID.