Why are pollen grains preserved as fossils?

The correct answer is Option (3) → Due to the presence of sporopollenin.

A fossil forms only when a structure can last for a very long time without breaking down. The outer wall of a pollen grain, the exine, is made of sporopollenin. So pollen grains survive in rocks for millions of years.

Option (1) Due to the presence of cellulose. — Incorrect. Cellulose is found in the intine, the thin inner wall of the pollen grain. It breaks down easily, so it cannot keep the grain intact as a fossil.

Option (2) Due to the presence of suberin and pectin. — Incorrect. Pectin is also part of the soft intine, and suberin is not named as a part of the pollen wall at all. Neither is resistant enough to make a fossil.

Option (3) Due to the presence of sporopollenin. — Correct. Sporopollenin in the exine resists heat, acids, alkalis and enzymes. This is why pollen grains are well preserved as fossils.

Option (4) Due to the presence of lignin. — Incorrect. Lignin hardens the walls of wood cells. It is not the material of the pollen wall.

NCERT Extract:
Stamen, Microsporangium and Pollen Grain – “… The hard outer layer called the exine is made up of sporopollenin which is one of the most resistant organic material known. It can withstand high temperatures and strong acids and alkali. No enzyme that degrades sporopollenin is so far known. … Pollen grains are well-preserved as fossils because of the presence of sporopollenin. … The inner wall of the pollen grain is called the intine. It is a thin and continuous layer made up of cellulose and pectin.”