A 0.050 mol dm⁻3 solution of a weak acid has a pH of 3.37. What is the value of the dissociation constant of the acid?

a) 1.8 x 10⁻⁶
b) 3.2 x 10⁻⁵
c) 6.7 x 10⁻⁴
d) 9.1 x 10⁻³