What Is the PH of a Buffer That Results When

What is the pH of a buffer that results when 0.50 mole of H₃PO₄ is mixed with 0.25 mole of NaOH and diluted with water to 1.00 L? (The acid dissociation constants of phosphoric acid are K_a1 = 7.5 × 10^-3,K_a2 = 6.2 × 10^-8,and K_a3 = 3.6 × 10^-13)

A) 1.82
B) 2.12
C) 6.91
D) 7.21
E) 12.44