Answer : Option D

Explanation :

Author: The author specifies style sheets for a source document according to the conventions of the document language. For instance, in HTML, style sheets may be included in the document or linked externally.
User: The user may be able to specify style information for a particular document. For example, the user may specify a file that contains a style sheet or the user agent may provide an interface that generates a user style sheet (or behaves as if it did).
User agent: Conforming user agents must apply a default style sheet. A user agent’s default style sheet should present the elements of the document language in ways that satisfy general presentation expectations for the document language