What is the monomer of natural rubber and its IUPAC name?

The monomer of natural rubber is isoprene. The IUPAC name for isoprene is 2-methylbuta-1,3-diene.

• Natural rubber is a polymer of isoprene.
• Isoprene has the chemical formula $\text{C}_5\text{H}_8$ and its structure is a four-carbon chain with two double bonds and a methyl group attached to the second carbon.

Let's examine the options:

• A. 1-buten-3-yne: This molecule contains both a double bond and a triple bond, and is not isoprene.
• B. 2-methylbuta-1,3-diene: This is the correct IUPAC name for isoprene, which is the monomer of natural rubber.
• C. 2-chlorobuta-1,3-diene: This is chloroprene, the monomer used to produce neoprene (a synthetic rubber), not natural rubber.
• D. buta-1,3-diene: This is a common monomer for synthetic rubbers like polybutadiene or styrene-butadiene rubber (SBR), but not natural rubber.

The final answer is $\boxed{\text{B}}$.