How does vulcanization change the properties of natural rubber ?

Question

$\begin{array}{1 1} \text{(a) Vulcanization is a chemical process,of heating rubber with Nitrogen.} \\\text{(b) Vulcanization is a chemical process, which is used to harden rubber} \\ \text{(c)Vulcanization is a chemical process, which is used to soften rubber} \\ \text{(d)Vulcanization is a chemical process, which is used to increases the production of rubber} \end{array}$