Which of the following does not belong to natural polymers?

Natural polymers are polymers that occur naturally in plants, animals, or living organisms.

Examples:

Cellulose → Found in plant cell walls
Starch → Plant storage polysaccharide
RNA → Natural nucleic acid polymer

Rayon is not a natural polymer. It is a semi-synthetic (artificial) polymer produced chemically from natural cellulose and is therefore known as artificial silk.

Hence, the correct answer is Rayon.