Hydrocyanic acid has a Ka of 4.0 x 10-10. What is the percent of
ionization of a 1.0 molar solution?
a. 0.004%
b. 0.0020%
c. 0.0004%
d. 0.02%

For a weak acid, we know that:

Ka=[H+][CN-]/[HCN];

Co=[HCN]+[CN-]; and

[H+]=[CN-].

Chemical Principles, fourth edition, by Dickerson, Gray, Darensbourg
and Darensbourg, The Benjamin/Cummings Publishing Company, Inc., 1984,
page 179

Therefore, Ka=[H+]^2/(Co-[H+])

and

[H+]^2+4.0 X 10^-10[H+]-4.0 X 10^-10=0.

Using the quadratic equation, we find that [H+] = 2 X 10^-5

The percent of ionization is calculated by [H+]/Co X 100 = 0.002%. 
Therefore, the answer is B.

Sincerely,

Wonko