1. Analyze the assertion

   The assertion says:

   Nitrogen and oxygen are the main components of the atmosphere but these do not react to form oxides of nitrogen.

   In ordinary atmospheric conditions, $\mathrm{N_2}$ and $\mathrm{O_2}$ do not react appreciably because the nitrogen molecule has a very strong triple bond: $\mathrm{N\equiv N}$ This makes nitrogen relatively inert at normal temperature.

   So, in the usual sense, the assertion is correct: although both gases are abundant in air, they do not combine under ordinary conditions to form nitrogen oxides.

2. Analyze the reason

   The reason says:

   The reaction between nitrogen and oxygen requires high temperature.

   This is correct. At high temperature, nitrogen and oxygen can react: $\mathrm{N_2 + O_2 \rightarrow 2NO}$ This occurs, for example, during lightning or in automobile engines, where very high temperatures are available.

3. Check whether the reason explains the assertion

   Yes. The assertion is true because under normal atmospheric conditions the temperature is not high enough to overcome the activation energy needed for reaction between $\mathrm{N_2}$ and $\mathrm{O_2}$.

   Thus, the reason correctly explains why nitrogen and oxygen normally do not react in the atmosphere.

4. Evaluate the options

   • A: Both assertion and reason are correct, and the reason is the correct explanation for the assertion. ✅
   • B: Both are correct, but reason is not the correct explanation. ❌
   • C: Assertion incorrect, reason correct. ❌
   • D: Both incorrect. ❌

Therefore, the correct option is A.