Natural rubber is a polymer of

2-methyl-1, 3-butadiene

The answer is 2. 2-methyl-1, 3-butadiene.

Natural rubber is a polymer of isoprene, which is a molecule with the chemical formula C5H8. Isoprene is a conjugated diene, which means it has two double bonds that are separated by a single bond. When isoprene molecules polymerize, they form a long chain with alternating single and double bonds. This type of polymer is called a cis-polyisoprene, and it is what gives natural rubber its characteristic elasticity.

The other options are not polymers of isoprene. 1, 1-dimethylbutadiene has three carbon atoms in the main chain, while isoprene has four. 2-chlorobuta-1, 3-diene has a chlorine atom in the main chain, while isoprene does not. 2-chlorobut-2-ene has a double bond in the middle of the main chain, while isoprene has two double bonds at the ends of the main chain.

So, natural rubber is a polymer of 2-methyl-1, 3-butadiene, not 1, 1-dimethylbutadiene, 2-chlorobuta-1, 3-diene, or 2-chlorobut-2-ene.