The correct option is D 2NO2⇌N2+2O2;Kc=1×1015
Smaller is the equilbrium constant (KC),
Slowere is the rate of reaction,
Slower is the rate of reaction implies that the reactant is more stable. In the given question Kc=1015 is the smallest equribibrium constant.
So, NO2 is the most stable oxide of nitrogen.