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

Sporopollenin is one of the most resistant biological materials, found in the exine of pollen grains. It is resistant to microbial and chemical decomposition and due to this, pollen grains are well preserved during fossilisation. So, the correct answer is option D (assuming options are A,B,C,D and Sporopollenin is option D).