The correct answer is b. The ideal gas law must be satisfied for both air parcels. Since the pressure is the same for each of them, the right-hand side of the equation must be the same for each of them, too.
Denser air is heavier than lighter air.
In the above example, the pressure beneath the denser column is higher than the pressure beneath the lighter column, since the pressures at the top of the columns are equal. Now, let's get back to the hydrostatic equation. Suppose you had two columns of air which featured exactly the same difference in air pressure between the top and bottom. They must be the same height. They must have the same average air density.