Assertion: Nitrogen is inactive in normal chemical reactions.
Reason: The bond dissociation energy is directly proportional to reactivity.
(A) Both assertion and reason are correct and reason is the correct explanation of the assertion.
(B) Both assertion and reason are correct but the reason is not the correct explanation of the assertion.
(C) Assertion is correct but the reason is incorrect
(D) Assertion is incorrect but reason is correct.

Hint: Nitrogen gas with nitrogen atoms is triply bonded. Bond dissociation enthalpies are always positive, since they represent the endothermic homolysis of a covalent bond. The bond-dissociation energy is one measure of the strength of a chemical bond A–B. It can be defined as the standard enthalpy change when A–B is cleaved by homolysis to give fragments A and B.

Complete answer:
Bond dissociation enthalpy can be defined as the standard change in enthalpy when a bond is cleaved via homolytic fission. The products formed from the homolysis of the bond are generally radicals.
The bond dissociation energy is inversely proportional to the reactivity of the compound. More is the bond dissociation energy of the compound, lesser will be its reactivity.
Nitrogen is inactive in normal chemical reactions because it is highly unreactive and has high bond dissociation energy. Hence Assertion is correct but the reason is incorrect.

Therefore, the correct option is the C option.

Note:
In nitrogen gas, Two nitrogen atoms are triply bonded to each other. Because of the triple bond, it has high bond dissociation energy. The bond energy of ${{N}_{2}}$ is 946 kJ per mole. ${{N}_{2}}$ have effective p-p orbital overlap, which makes it a stronger bond.