Which of the following is the strongest acid?

A

H3​PO4​

B

H2​SO4​

C

HNO2​

D

CH3​COOH

The more number of H+ ions an acid can release the stronger is the acid. So from the given acids, H3​PO4​ can release 3H+ ions. But H2​SO4​ is stronger than H3​PO4​ as in solution H2​SO4​ dissociates more than H3​PO4​ and we know strong acids dissociates completely in water whereas H3​PO4​ partially dissociates. Hence H2​SO4​ is the strongest acid.