Which of the following is strong acid?

Nitric acid completes dissociates in aqueous solution. So it releases more H₃O ⁺. Thus it is a strong acid.

Reaction: HNO₃ + H₂O → H₃O ⁺ + NO₃^-

If 1M Nitric acid is taken then it will completely dissociate in aqueous solution to give 1M H₃O ⁺.

Acetic acid, oxalic acid, and citric acid do not dissociate completely

in aqueous solution, thus they are weak acids.