When looking at the sentence, you can see that it mentions a lot of artisitic qualities. (sketch, come up with new pencil, complex shading, drawings)
He may be enthusiastic and engaged in his work, but it isn't the main point of the sentence. He is being very artistic, doing things that exclusive artists would do. It best describes what he's doing.
Simon is the most "A" <em>artisitic </em>person I know..... makes the most sense.
3 of these words would fit well in the blank. However, this sentence talks mostly about how well Simon sketches and how detailed he is in his art. Taking this into account, I believe the answer is A.) Artistic