Answer is (a) human adaptability.
Rationale for rejecting b,c,d options:
Humans do not have a high fecundity, they can give birth to 1-3 offspring on an average.
The skin tone variation is an adaptation that is based on the geography. The sunlight intensity affects it. It is a part of the ability of the humans to efficiently adapt to their surroundings.
The body variation in the form of physical features is also the result of the ability of the humans to adapt to their surroundings.