Which of the following is a guideline for using headings in content outlines?

A) Different font styles and colors should be used for headings of the same level.
B) First-level headings should contain only one second-level heading.
C) All first-level, second-level, and third-level heading should be numbered using Roman numerals.
D) All headings of the same level must be treated consistently.

It is important to maintain consistency in formatting and treatment of headings of the same level throughout the outline. This helps with the organization and clarity of the content. Using different font styles or colors for headings of the same level (option A) can be distracting and make it harder to understand the hierarchy of the content. First-level headings can have multiple second-level headings (option B) if it makes sense organizationally. Numbering headings using Roman numerals (option C) is not a standard convention and can be confusing for readers.