The least likely explanation for one individual being able to grow muscle faster and larger than the other is:
A. One individual was born with more muscle fibers.
While genetic factors and the number of muscle fibers can influence muscle growth potential, the other factors listed (consistent training schedule, healthier diet, and intense training schedule) are more directly related to how effectively one can stimulate muscle growth during a training program. Therefore, the option stating that one individual was born with more muscle fibers is the least likely explanation in the context of this particular training program comparison.