As relative molecular mass of molecule increases its rate of diffusion decreases . Mr of HCl is 1+35.5=36.5 which is greater than Mr of NH3 which is only 14+3=17. As HCl is heavier it'll diffuse at a lower rate so travel less distance whilst NH3 will cross a greater distance as it diffuses faster so white solid appears closer to HCl that is approximately B.
