(d)
The acidity of oxyacids of halogens increases with increase in oxidation state of halogen.
Oxidation state of Cl in HCIO=+1
Oxidation state of Cl in HCIO_{2}=+3
Oxidation state of Cl in HCIO_{3}=+5
Oxidation state of Cl in HCIO_{4}=+7
Hence, HCIO_{4} has highest acidity among oxyacids of chlorine.