Step 1: Understanding Lassaigne’s test.
Lassaigne’s test is used for detecting halogens, nitrogen, and sulfur in organic compounds. When an amine (like an aliphatic amine) is heated with sodium metal in fusion, sodium azide (NaN$_3$) is formed if nitrogen is present.
Step 2: Conclusion.
Upon subjecting an aliphatic amine to Lassaigne’s test, the product formed is sodium azide (NaN$_3$), corresponding to option (D).