Answer:
HClO4
Step-by-step explanation:
Of the four oxyacids with Cl as the central atom: HClO, HClO2, HClO3, and HClO4, HClO4 is the strongest acid. This is because once it loses its hydrogen, the central Cl will strongly pull electron density toward itself. This leaves us with a conjugate base that is more stable than the conjugate bases of HClO, HClO2, and HClO3. The strength of an oxyacid increases as the oxidation state of the central atom becomes larger.