Correct statements among the following is/are
a) NO₂ is a reddish brown paramagnetic gas but its dimer N₂O₄ is colorless diamagnetic solid.
b) NO₂ is mixed anhydride
c) N₂O₃ is mixed oxide
d) HNO₂ exists in two tautomeric forms

NO₂ is a brown color gas but its dimer which is N₂O₄ is a colorless solid.

Nitrogen dioxide a mixture of anhydride means the compound forms a mixture of two acidic anhydrides when reacts with water.

Dinitrogen trioxide is prepared by mixing equal parts of nitric oxide and nitrogen dioxide.

Nitrous acid exists in two tautomeric forms
(1) nitro form as in nitro alkanes
(2) nitrite form as in alkyl nitrite