Outer wall of pollen grain is formed of

Answer: Option D

Outer wall of pollen grain is formed of Sporopollenin.
Pollen grains have a hard coat made of sporopollenin that protects the gametophytes during the process of their movement from the stamens to the pistil of flowering plants, or from the male cone to the female cone of gymnosperms.
Each pollen grain contains vegetative cells and a generative (reproductive) cell. In flowering plants the vegetative tube cell produces the pollen tube, and the generative cell divides to form the two sperm nuclei.