The mixed anhydride of nitrogen is:

Mixed anhydride reacts with water to form a mixture of two acidic anhydride.

Two oxides of nitrogen are acid anhydrides i.e  they react with water to form two nitrogen-containing oxyacids.

2NO₂ + H2O  HNO₃ + HNO₂

Final answer : Hence, the correct option is A i.e, N₂O₄ (2NO₂)