Which of the following molecules is a carbohydrate?

Correct answer: D) Lactose.
Lactose is a disaccharide, a type of carbohydrate composed of two sugar molecules (glucose and galactose) bonded together. Carbohydrates are a class of biomolecules that include sugars, starches, and fibers, and are typically composed of carbon, hydrogen, and oxygen atoms.

The other options are not correct because:
Palmitic acid (A) is a fatty acid, a type of lipid molecule.
Collagen (B) is a protein, a type of biomolecule composed of amino acids.
Insulin (C) is a protein hormone, also composed of amino acids.
Note: Carbohydrates are an important source of energy for many living organisms, and are found in a wide range of foods, including fruits, grains, and dairy products.