The tertiary consumer will receive 500 calories.
<u>Explanation:</u>
Calories produced by the producer = 500,000
The flow of energy through different trophic levels is 10%
After producer comes the primary consumer followed by secondary consumer followed by tertiary consumer.
Energy reached to primary consumer = 
= 50000 calories
Energy reached to secondary consumer = 
= 5000 calories
Energy reached to tertiary consumer = 
= 500 calories
Therefore, the tertiary consumer will receive 500 calories.
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.