What is the definition of oxidation number?

Which of the following can show the disproportionation reaction?

${{\mathrm{ClO}}_4}^{-}, {\mathrm{Cl}}^{-}, {\mathrm{ClO}}_2^{-}, {\mathrm{ClO}}_3^{-}$

Find the oxidation number of phosphorous in the following compounds.

Phosphorus acid $\left({\mathrm{H}}_3{\mathrm{PO}}_3\right)$

Find the oxidation number of phosphorous in the following compounds.

Orthophosphoric acid $\left({\mathrm{H}}_3{\mathrm{PO}}_4\right)$

Find the oxidation number of phosphorous in the following compounds.

Metaphosphoric acid $\left({\mathrm{HPO}}_3\right)$