Final answer:
Fathom, Clearsight, and Indigo are the three dragons that betrayed Darkstalker in the Wings of Fire series, aligning with option (a). Therefore, the correct option is a) Fathom, Clearsight, and Indigo.
Step-by-step explanation:
The three dragons that betrayed Darkstalker were Fathom, Clearsight, and Indigo. This corresponds to option (a) Fathom, Clearsight, and Indigo. In the context of the Wings of Fire book series, these characters played crucial roles in the story and their actions had significant consequences for the plot. It is important to recognize that these events are part of the fictional world created by author Tui T. Sutherland in her fantasy novels centered around dragons.