(a) $HClO_2$ has the largest $K_a$.
Work Step by Step
The only difference between these 3 acids is the central atom. With a more electronegative central atom, the H-O bond gets more weak and polarizes, making the oxyacid more acidic. Since Chlorine is the most electronegative of these 3, $HClO_2$ is the stronger acid, which means the largest $K_a$.