Answer: Jessica's mom is clearly referring to "SET-POINT THEORY".
Therefore option E is most correct
Explanation: Set point is the amount of weight the body needs, for it to function optimally. Set point theory states that, a person's body will fight to maintain a certain range of weight for the body to function properly. This explains why some people eat more, but still look skinny, while some people eat less, but the body is not slimming down.
The mom was trying to encourage her, that the body has been set to that weight range, and she can never change it, so it's not her fault. The mother understood set- point theory, and has used it to comfort Jessica.