Which of the following has proved helpful in preserving pollen as fossils?

The correct option is D Sporopollenin
Sporopollenin is one of the most chemically inert biological polymers. It is a major component of the tough outer (exine) walls of plant spores and pollen grains.
They are probably the most resistant organic materials of direct biological origin found in nature and in geological samples” as intact microcapsules found in ancient sedimentary rocks.