The class attribute a. can be applied to only one element in an HTML document b. determines the copy that will be displayed when the mouse hovers over an element c. can be used by CSS to apply the same formatting to more than one HTML element d. determines the priority level of an HTML element

The answer is likely C. "Can be used by CSS to apply the same formatting to more than one HTML element."

Explanation:

Given that the name of the attribute is "class," it sounds like the kind of code that would be applied to multiple elements intended to display something a specific way.

Qualification:

I've invested hours of my time into learning HTML, but I've never dealt with the "class" attribute. So, maybe take my answer as a grain of salt.