What is meant by disproportionation of an oxidation state ? Give an example

Question

$\begin{array}{1 1}\text{(a) If oxidation state decrease in one reaction.}\\\text{(b) If oxidation state increases in one reaction.}\\\text{(c) If oxidation state increase and decrease simultaneously in one reaction}\\\text{(d) If oxidation state becomes equal in one reaction.}\end{array}$