Under similar conditions of pressure and temperature, 40 ml of slightly moist hydrogen chloride gas is mixed with 20 ml of ammonia gas,the final volume of gas at the same temperature and pressure will be

The correct option is B 20 ml
$N{H}_3$ + $HCl$ $⟶$ $N{H}_{4}Cl$

According to the given question, 1 ml $N{H}_3$ reacts with 1 ml HCl to produce 1 ml of $N{H}_{4}Cl$.

So, 20 ml of ammonia reacts with 20 ml of HCl to produce 20 ml of resultant $N{H}_{4}Cl$. The rest of HCl will stay unreacted.

So, the correct option is '20ml'.