How do you define a class selector in CSS?

A class selector in CSS is defined by using a period (.) before the class name, such as .classname. This syntax allows you to apply styles to all HTML elements that have the specified class attribute. When you define styles this way, every element marked with the class in the HTML can be styled consistently and efficiently.

For instance, if you have several <div> or <span> elements that share the same class, applying the styles will ensure that all of them reflect the same design properties, such as color, font size, margins, etc. This enhances maintainability in your code, making it easy to apply changes universally to elements with that class.

The other options describe different selectors: a hash (#) indicates an ID selector, which targets a specific element with that ID; a colon (:) is used for pseudo-classes, which apply styles based on the state of an element (like :hover); and an asterisk (*) denotes the universal selector, which applies styles to all elements. These serve different purposes and are important in their own contexts, but they do not correctly define a class selector.