Final answer:
To understand the meaning of the adjective 'saturnine' as used by an author to describe a character, consulting a dictionary would be the best choice as it provides word definitions.
Step-by-step explanation:
If an author uses the adjective saturnine to describe a character, the reference that would best explain what this means is D. A dictionary. A dictionary provides definitions of words, detailing their meanings, pronunciations, origins, and grammatical information. When looking to understand what a particular adjective means in the context of character description, a dictionary is the most direct and helpful resource.