How is a class defined in CSS?

In CSS, a class is defined using a period (.) followed by the class name. This syntax indicates to the browser that specific styles should be applied to any HTML element that has that class attribute assigned to it. For instance, if you define a class in your stylesheet as .highlight { background-color: yellow; }, any HTML element with the class attribute class="highlight" will have a yellow background.

Using a period is a key convention in CSS that differentiates classes from IDs and other selectors. Classes allow you to apply styles to multiple elements on a web page, facilitating a modular and reusable approach to styling. This is particularly useful when you want to maintain consistency across various elements without creating repetitive styles for each.

The other options represent incorrect syntax for CSS class declarations, which would not be recognized by browsers and therefore would not achieve the intended styling. Understanding the correct way to define and use classes in CSS is essential for effective web design and development.