What does 'Bu' & 'Na' & 'S' stand for in Buna-S polymer respectively?

Question

$\begin{array}{1 1}(a)\;\text{1,3-butadiene,styrene,sodium}\\(b)\; \text{butadiene,sodium,styrene}\\(c)\;\text{benzene,sodium,styrene}\\(d)\;\text{butyl alchol,sodium,styrene}\end{array}$