The correct answer is A. One individual was born with more muscle fibers.
If two individuals start the same training program at the same time and one is able to grow muscle faster and larger than the other one then the main reasons can be all other options as a more consistent training schedule leads you off to better results.
Similarly, the healthier the diet is the better muscle mass you gain and diet affects a lot.
Lastly, your training schedule affects the weight gain too and might lead oyu better off.