Formic Acid Is a Weak Acid Naturally Occurring in Certain $\times$

Formic acid is a weak acid naturally occurring in certain species of ants. Calculate the acid ionization constant for formic acid if a 0.213 M solution is 2.86% ionized. The abbreviated structural formula for formic acid is HCOOH.

A) 7.2 $\times$ 10^-4
B) 6.4 $\times$ 10^-4
C) 1.8 $\times$ 10^-4
D) 1.9 $\times$ 10^-5
E) 3.5 $\times$ 10^-8