In the air, N₂ and O₂ occur naturally but they do not react to form oxides of nitrogen because

The correct option is "the reaction is endothermic".

To understand why nitrogen ($N_2$) and oxygen ($O_2$) do not react at ordinary temperatures to form nitrogen oxides, we need to look at the chemical bonding and thermodynamics of the reaction.

1. Strong Triple Bond in Nitrogen:
Nitrogen gas exists as diatomic molecules ($N_2$) where the two nitrogen atoms are held together by a exceptionally strong triple covalent bond ($N\equiv N$). Breaking this bond requires a huge amount of energy (bond dissociation enthalpy of approximately 941 kJ/mol). Oxygen ($O_2$) also possesses a strong double bond ($O=O$) with a bond dissociation enthalpy of about 498 kJ/mol.

2. Endothermic Nature of the Reaction:
Because of the immense energy required to break these strong reactant bonds, the formation of nitric oxide ($NO$) from elemental nitrogen and oxygen is a highly endothermic reaction. The chemical equation representing this is:
$N_2+O_2\to 2NO$ (where $\Delta H>0$)

3. Why They Do Not React Naturally in Air:
Under normal atmospheric conditions (room temperature and pressure), the ambient thermal energy is far too low to overcome the high activation energy barrier presented by these strong bonds. Since the reaction is strongly endothermic, it requires extremely high temperatures (above 2000°C)—such as those found during lightning strikes or inside internal combustion engines—to initiate and proceed.