Dinitrogen is a robust compound, but reacts at high altitude to form oxides. The oxide of nitrogen that can damage plant leaves and retard photosynthesis is

We are asked to identify the oxide of nitrogen that damages plant leaves and retards photosynthesis.

At high altitudes, the high temperature from lightning or combustion causes nitrogen and oxygen to combine, forming various nitrogen oxides. Among these, nitrogen dioxide ($$NO_2$$) is a reddish-brown, pungent gas that is a major air pollutant.

$$NO_2$$ is particularly harmful to vegetation because it dissolves in the moisture on leaf surfaces to form nitric acid ($$HNO_3$$) and nitrous acid ($$HNO_2$$). These acids damage the leaf tissue, destroy chlorophyll, and thereby retard the process of photosynthesis. This leads to visible symptoms such as leaf bleaching and necrosis.

Now, NO (nitric oxide, Option A) is a colourless gas that is less directly toxic to plants compared to $$NO_2$$; it is primarily a precursor that gets oxidised to $$NO_2$$ in the atmosphere. $$NO_3^-$$ (Option B) is the nitrate ion, which is actually a plant nutrient, not a pollutant that damages leaves. $$NO_2^-$$ (Option D) is the nitrite ion, which exists in solution form and is not an atmospheric pollutant that directly damages plant leaves.

Hence, the correct answer is Option C.