The correct answer is C. sulphur.
Sulphur is added to natural rubber to make it strong and more bouncy. This is done through a process called vulcanization. Vulcanization is a chemical process that makes rubber more durable and resistant to heat, cold, and wear. It is also used to improve the elasticity and tensile strength of rubber.
Polythene, sponge, and chlorine are not added to natural rubber to make it strong and more bouncy. Polythene is a type of plastic that is used to make a variety of products, including bags, bottles, and pipes. Sponge is a soft, porous material that is used for cleaning and insulation. Chlorine is a chemical element that is used to disinfect water and to produce a variety of chemicals.