The correct option is B 4M
According to the relationship:
Molar mass=2×vapour density
We have given mass of B is M, thus vapour density of B is,
V.D.=M (Numerical value)2
Also, given that vapour density of A is four time that of B, then
Therefore, V.D. of A = 4×V.D.
MA2=4×MB2
MA=4MB=4M