The correct option is C sporopollenin
Pollen grain consists of two coats. Exine is the hard outer coat made of sporopollenin, which is one of the most resistant organic material. Sporopollenin helps the pollen to withstand high temperatures, strongly acidic and alkaline conditions. There is no known enzyme that can degrade it. So it is highly stable and tough.
Intine is a thin inner coat made of cellulose and pectin.