compound on analysis was found to contain $8\%$ sulphur by mass. The least molar mass of that compound will be.

$8\%$ sulphur in a compound means that :

$8 \mathrm{g}$ sulphur is present in a compound $= 100 \mathrm{g}$

$\therefore 32 \mathrm{g}$ sulphur is present in a compound $=\frac{100}{8}x 32$

                                                                    $= 400 \mathrm{g}$ ($\therefore$at. wt. of $\mathrm{S}=32$)

Thus, the least molar mass, a compound containing only one sulphur atom will be $400 \mathrm{a}.\mathrm{m}.\mathrm{u}.$ So, the correct answer is $\left(\mathrm{c}\right).$