The catabolic metabolism has 3 stages.
The first stage of catabolic metabolism is called "Hydrolysis" . It is the digestion in the lumen of the gastrointestinal tract. Carbohydrates, fats, and proteins are broken down into their individual monomer units, simple sugars, fatty acids and glycerol, and amino acids, respectively.
The correct answer is B: stomach.
The first stage of catabolic metabolism occurs in the stomach.