The __________ 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.

The correct answer is: A. author

The author is the person who creates the source document. They are responsible for specifying the style sheets for the document, according to the conventions of the document language. For instance, in HTML, style sheets may be included in the document or linked externally.

The user is the person who reads or interacts with the source document. They do not specify the style sheets for the document.

The user-agent is a software agent that interacts with a user on behalf of another application. They do not specify the style sheets for the document.

None of the other options are correct.