Assertion (A): Benzene diazonium chloride does not give Lassaigne's test for nitrogen. 

Reason (R): Nitrogen is lost as N₂ gas on heating.

Lassaigne's test is also known as the sodium fusion test. It is used in the elemental analysis of organic compounds.

Organic compound is fused with sodium and the carbon and nitrogen present in the compound form $\mathrm{NaCN}$ which is used in the test.

Benzene diazonium chloride doesn't form $\mathrm{NaCN}$ as it loses ${\mathrm{N}}_2$ gas on heating.

Thus, both assertion and reason are true and reason is the correct explanation of assertion

Hence, option (A) is correct.