To determine which action contributes the MOST to the theme of the story, it would depend on the broader context and themes present in the story. However, if we consider common themes such as family, support, and personal growth, the option that stands out is:
C. her encouraging and guiding Prim.
This action likely emphasizes themes of mentorship, support, and the importance of relationships. Encouraging and guiding Prim suggests a focus on personal connections and the growth that can come from them, which is often central to many narratives.