Concept: Joint demand occurs when two or more commodities are needed together to satisfy a single want.
Explanation: Such goods are complementary, meaning they are used side by side.
Examples include cars and petrol, pens and ink, and tea and sugar.
When the want is one, but satisfying it requires more than one commodity, the demand for those commodities is called joint demand.
This differs from composite demand (one commodity serving many uses), competitive demand (goods competing for the same want), and derived demand (demand based on another good's demand).
Answer: D. Joint