What is the other name of the monomer of natural rubber?

Question

$\begin{array}{1 1}(a)\;\text{2-Methyl buta-1,3-diene}\\(b)\; \text{2-Methyl buta-1-ene}\\(c)\;\text{Diethyl buta-1,3-diene}\\(d)\;\text{2-Methyl buta-3-ene}\end{array}$