The vapour density of gas A is four times that of B. If the molecular mass of B is $M$, then the molecular mass of A will be:

The correct option is B $4M$
According to the relationship:
$\text{Molar mass}=2\times \text{vapour density}$
We have given mass of $B$ is $M$, thus vapour density of B is,
$\text{V.D.}=\frac{\text{M (Numerical value)}}{2}$
Also, given that vapour density of A is four time that of B, then
Therefore, V.D. of A = $4\times \text{V.D.}$
$\frac{M_A}{2}=4\times \frac{M_B}{2}$
$M_A=4M_B=4M$