Answer:
c. Mitochondria
Explanation:
Mitochondria are the double membrane-bound structures and are the site for aerobic cellular respiration. The process of aerobic cellular respiration produces a large amount of ATPs which are used by cells for other functions.
As per given information, Billy is becoming weaker and experiences fatigue due to lack of ATP through aerobic cellular respiration, his mitochondria in his muscle cells may not be functioning properly. lack of ATPs might be due to abnormal mitochondrial functioning.