The first paragraph of the passage introduces Sam Gilliam and his work, giving a brief overview of what it's like. The next paragraph goes into Gilliam's motivations, and the last discusses what the work actually looks like - in other words, the nature of the work. Thus, we can see that the author's primary purpose is exactly what (A) describes.
(C), on the other hand, is a good description of the last paragraph, but leaves out important descriptions of Gilliam's motivations introduced in the second. (A) is simply more comprehensive than (C) is, why is why it's a better answer. Hope this helps!
