Final answer:
The correct answer is c. required fields. CSS3 pseudo-classes like :valid and :invalid style form fields based on validation status, but there's no pseudo-class solely for required fields, making 'required fields' the correct answer.
Step-by-step explanation:
CSS3 pseudo-classes are used to select and style elements based on their state or position in the document tree. While CSS3 includes pseudo-classes such as :valid and :invalid, which apply styles to form fields based on their validation status, no pseudo-class exists for directly styling required fields solely based on the required attribute. In contrast, the :optional pseudo-class can be used to style non-required, or optional, fields. This highlights the lack of a direct pseudo-class for required fields in CSS3, thus making 'required fields' the correct answer.
The correct answer is c. required fields. CSS3 pseudo-classes are used to select and style elements based on their state or position in the document tree. For example, the :valid and :invalid pseudo-classes are used to style form fields based on their validation status. The :required and :optional pseudo-classes can be used to select and style required or non-required form fields. However, CSS3 pseudo-classes cannot be used to directly format required fields. Therefore, the correct answer is c. required fields that cannot be formatted using CSS3 pseudo-classes.