Which of the following has the highest concentration of H+ ions in aqueous solutions?

Question

The following are the answers out of the book.
a. .30 M H2CO3
b. .20 M HCl
c. .50 M HCOOH
d. 1.0 M NH3

What i need to know is why they are choosing .20 HCl as the answer.

Answer 1

Although it's the most dilute, HCl is a strong acid, providing 100% of its H as H+ ions. The other acids don't do this, and NH3 is an alkali!