Final answer:
Reagan's transformation and tails primarily build the plot by creating conflict and tension, which is a key part of the rising action and essential for driving the story forward.
Step-by-step explanation:
The concept of Reagan’s transformation and tails (potentially referencing a shift in character or plot elements within a story) serve to build the plot primarily by creating conflict and tension. This aspect of the narrative aligns with the rising action, where characters encounter various challenges and set forth on journeys that introduce secondary conflicts. Conflict is essential as it is synonymous with the reason for telling the story—it is the driving force that engages readers and creates a buildup to the story's climax.
Moreover, plot development can include transitions that may introduce new characters or ideas, enhance tension, or provide connections to the overarching theme. Incorporating such dynamic changes within a story, be they transformative moments for characters like Reagan or plot mechanisms like 'tails,' contributes significantly to the narrative arc and sustains reader interest.