The molar mass of a substance is $20 g$. The molecular mass of the substance is

Molar mass is the mass of the avogadro number $\left(N_A\right)$ of molecules of that substance.

Numerically, molar mass = molecular mass.

but, molecular mass is expressed in atomic mass unit $(a.m.u)$.

molecular mass $=20$ amu

Hence, option (B) is correct.