I do not understand how to do this question

Ammonia is a base and concentrations of ammonia is important for pH calculations. Here pH is determined by calculation of [OH-] by making rice table and using Kb value.
K_b = [OH^-][NH₄⁺]/[NH₃]
[OH^-]= K_b[NH₃/[NH₄⁺]
so as  decreases  also decreases and pOH increases and hence pH decreases.
So B is the right option.