Calculate the ph of a 0.30 m naf solution. the ka value for hf is 7.2*10^-4

We are given with the equilibrium constant of acid, HF and is asked to calculate the pH of 0.30 M NaF solution. The formula to be followed is 
Ka = [H+][F-]/[HF]Ka = 7.2 x 10 -4 = x^2/[0.3-x]x = [H+]= pH = -log (H+) = 1.84

The correct answer is b

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.