Concept:Joint demand occurs when two or more goods are demanded together to satisfy a single need.
Explanation:A fall in the demand for product
X leads to a fall in the demand for product
Y.
This shows that
X and
Y are used together as complementary goods.
For example, cars and petrol are jointly demanded. If the demand for cars decreases, the demand for petrol also decreases.
Derived demand refers to a good whose demand depends on the demand for another good, such as labour.
Composite demand means a good demanded for several alternative uses.
Competitive demand applies to substitute goods, not complementary goods.
Thus, the relation between
X and
Y is a clear case of joint demand.
Answer:Joint demand (Option C).