How do class selectors begin in CSS?

Class selectors in CSS begin with a period, which is used to indicate that the style rules being defined apply to a specific class. For example, if you want to style all elements with the class "example", you would write it in your CSS as follows:

.example {

color: blue;

}

In this instance, the period before "example" denotes that it is a class selector. When this CSS is applied, any HTML element assigned the class "example" will have its text color changed to blue.

The other options, while they represent different types of selectors or syntax in CSS, do not apply to class selectors. A hyphen is generally used within names for CSS rules but does not start a selector, a comma is used for grouping selectors together, and a pound sign is used for ID selectors, which designate a unique identifier for HTML elements. Understanding these distinctions is crucial for effectively using CSS in web development.