Hint: One of the most resistant biological materials is the one that is very stable and preserved in soils and sediments. These are resistant to chemical degradation both from inorganic and organic chemicals. It is the most inert biological material and is present in the tougher outer wall of plant spores and pollen grains.
Sporopollenin is the most resistant biological material as it is very stable and chemically inert. Its composition is still elusive and is preserved in soils and sediments. It is present in the tough outer wall of plant spores and helps the cell wall to make resistance against any chemical or enzyme which can break cell spores of the plants. It is also present in pollen grains of the flower and helps to withstand pollen grains against any harsh conditions. Sporopollenin is resistant to enzymes and strong chemicals as it is very stable and even does not decompose in the soil after hundreds of years. Recent studies reveal that it consists of a mixture of biopolymers, containing phenylpropanoids, phenolics, fatty acids of long chains, and traces of carotenoids. Due to this property of sporopollenin, pollen grains are preserved during fossilization.
Hence, the correct option is C. Sporopollenin.
Note: Exine of pollen grains contains sporopollenin which helps the pollen to resist various chemical and microbial decomposition. Therefore, pollen grains are also found in fossils of hundreds of years old with decomposition. Tapetal cells surrounding the pollen grains secrete lipophilic globules and sporopollenin is present in these globules.