The correct option is C Sporopollenin
The outer wall of the pollen grain is called exine. It is composed of sporopollenin.
Sporopollenin is one of the most resistant materials. It cannot be degraded by high temperature, strong acids and alkalis. Also there are no enzymes which are capable of degrading sporopollenin. Thus the presence of sporopollenin in the outer wall of the pollen grains helps in preserving the pollen as fossils.
Cellulose, chitin and lignin undergo degradation.