Based on the description provided in the passage from "The Crow-Child," the best response that describes Ruky’s role in the story is:
He is a static character that helps move the plot forward with his motivations and actions.
Ruky is depicted as a young boy who is closely bonded with his sister, Cora, and while he may have his own experiences, his character does not seem to undergo significant change. Instead, he contributes to the story's progression through his interactions and relationship with Cora.