Introduction:
Sporopollenin is a chemically inert, complex biopolymer that is found in the outer walls of pollen grains and spores of land plants. It is one of the most resistant organic compounds known, capable of withstanding high temperatures, strong acids, and enzymes. Sporopollenin is responsible for protecting the reproductive cells of plants during their dispersal and germination.

Chemical Composition:
Sporopollenin is primarily composed of long-chain fatty acids and phenolic compounds. It is a highly cross-linked polymer, with a complex and irregular structure. The exact composition of sporopollenin can vary among different plant species, but it is predominantly made up of hydrophobic components.

Explanation:
The correct answer is option B, "Fatty substance". Sporopollenin is not a homopolysaccharide (option A), which refers to a polysaccharide composed of a single type of sugar unit. It is also not a protein (option C), which is a macromolecule composed of amino acids. While sporopollenin does contain some carbohydrates, it is primarily composed of fatty substances.

Key Points:
- Sporopollenin is a chemically inert biopolymer found in the outer walls of pollen grains and spores.
- It is highly resistant to various environmental conditions and protects plant reproductive cells.
- Sporopollenin is primarily composed of long-chain fatty acids and phenolic compounds.
- It has a complex and irregular structure, with a predominance of hydrophobic components.
- Sporopollenin is not a homopolysaccharide or a protein.
- The correct answer is option B, "Fatty substance".