Not 100% Sure but I think so
Answer:
exercising regularly, finding a purpose in life, having high self-esteem, and eating well
Explanation:
It’s D for sure, it’s the only signifiant, important and correct answer
I believe the answer is: holding genetics constant while varying the environment.
This could be done by making the identical twins grow up in a different environment. If the behaviour that they displayed is exactly the same even if they grow up in a different environment, the only explanation for that is their behaviour is influenced by genetics, and vice versa.