Diethylamine on oxidation with ${\mathrm{KMnO}}_4$ gives. 

• ${\mathrm{KMnO}}_4$ Potassium permanganate is a good oxidizing agent. 
• Amines undergo oxidation in the presence of different oxidizing agents to yield various products. The products mostly depend on the oxidizing agents used.

Diethylamine is a secondary amine which when oxidized in the presence of a strong oxidizing agent, ${\mathrm{KMnO}}_4$ yields Tetraethyl hydrazine.

Therefore, option (D) is correct.