Which of the following is redox reaction

A redox reaction is a reaction in which oxidation and reduction takes place simultaneously.

(A)

$2\mathrm{NaOH}+{\mathrm{H}}_2{\mathrm{SO}}_4⟶{\mathrm{Na}}_2{\mathrm{SO}}_4+2{\mathrm{H}}_2\mathrm{O}$

It is an acid base neutralization reaction which is not a redox reaction.

(B)

$3{\mathrm{O}}_2\to 2{\mathrm{O}}_3$. This is not a redox reaction

(C)

${\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$ in this reaction only the state of water changes.

(D)

${\mathrm{N}}_2+{\mathrm{O}}_2\to 2\mathrm{NO}$

Here the oxidation number on N changes from 0 to +2 and on O changes from 0 to -2. Thus N being reduced and O being oxidised confirm it to be the redox reaction.