c
Phosphorous acids contain \(P\) in \(+ 3\) oxidation state.

\(\begin{array}{l|c|c} \hline {\text { Pyrophosphorous acid }} & {\mathrm{H}_{4} \mathrm{P}_{2} \mathrm{O}_{5}} & {+3} \\\hline {\text { Pyrophophoric acid }} & {\mathrm{H}_{4} \mathrm{P}_{2} \mathrm{O}_{7}} & {+5} \\ \hline \text { Orthophosphorous acid } & {\mathrm{H}_{3} \mathrm{PO}_{3}} & {+3} \\ \hline \text { Hypophosphoric acid } & {\mathrm{H}_{4} \mathrm{P}_{2} \mathrm{O}_{6}} & {+4} \\ \hline\end{array}\)