Step 1: Recall the structure of pollen.
Exine is the outer layer of the pollen grain and is mainly composed of a highly resistant biopolymer.
Step 2: Identify the correct compound.
- Cellulose: Found in plant cell walls, but not in exine.
- Pectin: A structural polysaccharide but not the major component of exine.
- Sporopollenin: A tough, inert substance found in the exine layer. It is highly resistant to degradation.
- Xanthophyll: A pigment, not related to exine composition.
Step 3: Conclude.
Sporopollenin is the main component of the exine layer of pollen grain.
Final Answer:
\[
\boxed{\text{Sporopollenin}}
\]