Lets think about this for a second,
We should use the process of elimination with these type of questions.
A- I do not think this is correct, for it doesn't make that much sense.
B- You know the saying? "Money cannot buy happiness." I doubt it is B
C- Possibly, but there are an amount of people not considered gifted with beauty that are still happy.
Which leaves us with D. And if you read D. it would make the most sense out of all of them, right? At least I think so.
So your final answer is D. It is something people can cultivate through behavior.