The density of a gas is 2, relative to nitrogen, under the same conditions. The molecular weight of the gas is

 Using the relation, $PM=\rho RT$,  $\rho$ is directly proportional to M.

We  can also write the proportionality as $\frac{M_1}{M_2}=\frac{{\rho }_1}{{\rho }_2}$

Given, ${\mathrm{d}}_{\mathrm{gas}}=2{\mathrm{d}}_{{\mathrm{N}}_2}, \mathrm{i}.\mathrm{e}. {\mathrm{\rho }}_2=2{\mathrm{\rho }}_1$

Therefore, $M_2=M_1\frac{{\rho }_2}{{\rho }_1}$

From the given data, $M_2=28X\frac{2{\rho }_1}{{\rho }_1}$

Therefore $M_2$ = 56 grams

Hence the correct option is (C).