Which CSS selector defines an element with a class attribute equal to subject?

The correct choice is the one that uses the period (.) prefix, which represents a class selector in CSS. In CSS, a class selector is used to target elements that have a specific class attribute defined in their HTML. When you see .subject, this indicates that any HTML element with the class attribute set to "subject" will be selected for styling purposes.

For example, if you have an HTML element like <div class="subject">, applying styles using the .subject selector will affect this element. This is distinct from the other options, which either denote different types of selectors or do not relate to class attributes. The hash symbol (#) is used for ID selectors, representing a unique element, while the at symbol (@) suggests an at-rule in CSS (but does not function as a selector), and the double colon (::) is used for pseudo-elements.

Therefore, the class selector is the appropriate way to target elements with a class of "subject," making it the correct answer.