Butyl rubber has the monomer units

Question

$\begin{array}{1 1}(a)\;\text{Isobutylene+Isoprene}\\(b)\; \text{Isobutylene+Butadiene}\\(c)\;\text{Butadiene+Isoprene}\\(d)\;\text{Isobutylene+Sodium polysulphide}\end{array}$