In the story “The Crow-Child” by Mary Mapes Dodge, Cora's motivations primarily revolve around her relationship with her brother and her desire to get him back. While her desire to be a better sister does influence her actions, the main plot is set in motion by her efforts to recover her brother. Therefore, the most accurate choice that reflects Cora's motivations and how they advance the plot is:
C. Her desire to get her brother back leads to actions that resolve the plot.