The correct sequence of decreasing number of $\pi$-bonds in the structures of ${\mathrm{H}}_2{\mathrm{SO}}_3,{\mathrm{H}}_2{\mathrm{SO}}_4$ and ${\mathrm{H}}_2{ \mathrm{S}}_2{\mathrm{O}}_7$ molecules is

The correct sequence of decreasing number of pi-bonds is ${\mathrm{H}}_2{ \mathrm{S}}_2{\mathrm{O}}_7\left(4\right)>{\mathrm{H}}_2{\mathrm{SO}}_4\left(2\right)>{\mathrm{H}}_2{\mathrm{SO}}_3\left(1\right)$. The numbers in brackets indicate the number of pi bonds.

${\mathrm{H}}_2{\mathrm{SO}}_3 {\mathrm{H}}_2{\mathrm{SO}}_4$

${\mathrm{H}}_2{ \mathrm{S}}_2{\mathrm{O}}_7$

Hence the correct answer is (D) ${\mathrm{H}}_2{ \mathrm{S}}_2{\mathrm{O}}_7>{\mathrm{H}}_2{\mathrm{SO}}_4>{\mathrm{H}}_2{\mathrm{SO}}_3$.