The amount of energy a person can consume throughout the day without gaining weight is extremely dependent on
a. hunger.
b. appetite.
c. culture.
d. activity level.

I'm quite sure it's activity level. One other thing that could've been said is metabolism, but that isn't there. However, food is consumed for the body to have energy, and without that energy we wouldn't be able to function. And when we let that intake of food settle, it's stored as fat. But because of activity level, we are capable of burning those calories. 

D is the answer you are looking for. 

I’m guessing you’re more alert to yourself or your surroundings if you’re more concentrated

Decomposition is the cycle at which bacteria breaks down any organic material into the soil