The best way to gain weight is to increase your intake.
a. protein
b. mineral
c. fat
d. carbohydrate

The best answer is D.

Carbohydrates get  rapidly digested and are quickly converted to glucose . Once glucose levels in the blood rise, insulin that is produced by the pancreas begins to remove excess glucose from the blood and  stimulates the liver to turn it into a type of starch called glycogen, which is stored in the liver and muscles.

However the body can only store a limited amount of glycogen so the excess glucose is stored as body fat (adipose tissue). This is the process of putting on weight.