Assertion: Nitrogen and Oxygen are the main components in the atmosphere but these do not react to form oxides of nitrogen.
Reason: The reaction between nitrogen and oxygen requires high temperature.

We begin by examining the Assertion: “Nitrogen and Oxygen are the main components in the atmosphere but these do not react to form oxides of nitrogen.” Atmospheric air indeed contains roughly $$78\%$$ nitrogen as $$N_2$$ and about $$21\%$$ oxygen as $$O_2$$. Under ordinary atmospheric conditions (room temperature, normal pressure) these two gases coexist without undergoing any noticeable chemical reaction to give oxides such as $$NO$$ or $$NO_2$$. This statement is therefore correct.

Next we look at the Reason: “The reaction between nitrogen and oxygen requires high temperature.” The conversion of $$N_2$$ and $$O_2$$ into nitric oxide may be written as

$$N_2(g) + O_2(g) \rightarrow 2\,NO(g)$$

For this reaction to proceed, the very strong triple bond in $$N_2$$ (bond dissociation energy approximately $$945\ \text{kJ mol}^{-1}$$) must be broken. Breaking such a bond demands a large amount of energy, so a high activation energy barrier exists. As a consequence, an appreciable rate of reaction is observed only when the temperature is very high, for example during lightning discharges in the atmosphere or in internal-combustion engines where flame temperatures reach several thousand kelvin. Thus, the Reason is also a correct statement.

Now we must decide whether the Reason actually explains the Assertion. The Assertion states that oxides of nitrogen are not formed under normal atmospheric conditions. The Reason tells us that formation of these oxides needs high temperature. Because the usual atmospheric temperature is far below the required temperature threshold, the reaction hardly proceeds, which directly accounts for the observed lack of oxides. Therefore, the Reason supplies the genuine cause for the Assertion.

Both statements are true, and the Reason is the correct explanation for the Assertion. Hence, the correct answer is Option B.