Correct option is: (d) HClO4
The relative strength of an acid can be determined from one of three factors the number of protonizable hydrogens the number of oxygen atoms on the central atom and the electronegativity of the central atom. Since the identity of the central atom is the same chlorine as is the number of hydrogens compare the number of oxygen atoms. The HClO4 has the most oxygen atoms so it would have to be the strongest acid.
