The correct option is D.
In essay writing, an outline refers to a framework, on which one builds one's writing. Writing an outline usually involves writing out the main ideas and the sub ideas that one wants to present in a piece of writing. To do this, roman numeral is often used to indicate the main ideas in an outline. Generally, in formal outline, roman numeral is used to depicts the main ideas, capital letters are used to indicate the major details while Arabic numbers are used to depict the minor details. The major details are those details that support the main ideas while the minor details are those details that support the major details.