You will not gain weight from eating
a. only carbohydrates.
b. a substantial dinner.
c. only protein.
d. a substantial lunch.

Explanation:

There are different foods that you can eat without getting weight. Basically, people eat until they feel full, this satiety is achieved better eating rich protein food. The strategy is to feel satiated eating the less calories possible, like cakes for example.

So, basically eating only protein, you won't get weight, because you are getting satiated with no calories.

answer: i would say the brain but it seem that you have a brain.