A 010 M Solution of a Weak Monoprotic Acid Has a a Hydronium-Ion

A 0.10 M solution of a weak monoprotic acid has a hydronium-ion concentration of 4.6 $\times$ 10^-4 M.What is the acid-ionization constant,K_a,for this acid?

A) 2.1 $\times$ 10^-2
B) 3.2 $\times$ 10^-3
C) 4.6 $\times$ 10^-4
D) 2.1 $\times$ 10^-6
E) 5.5 $\times$ 10^-5