The expression converting molality (m) into molarity (M) is 

where r and ${\mathrm{M}}_2$ are density of solution and molar mass of the solute, respectively.

The expression converting molality (m) into molarity (M) is, $m=M/\left(\rho -M{M}_2\right)$

Where, M is the molality and m is the mass.

$\rho$ is density.

Hence,  option b is correct.