Answer:
The correct option is A) increase in atmospheric O2
Explanation:
The nerve cells and the muscle cells suggest that the the presence of oxygen in the atmosphere was an adaptive radiation which brought the existence of different animals. The conditions became favorable for different animals to survive on Earth only when the availability of oxygen improved in the atmosphere. As animal cells like the muscle cells require a constant supply of oxygen, we can hence say that increase in atmospheric oxygen was the adaptive radiation for supporting many group of animals.