Joint demand refers to:

Joint demand occurs when two or more goods are consumed together because they complement each other in fulfilling a need or want. For instance, if the demand for cars increases, there is often a corresponding increase in the demand for gasoline, as both products are used in conjunction. This relationship highlights how the rise in demand for one product directly affects the demand for another, reinforcing the idea of complementarity between them.

The other options do not capture this relationship effectively. The first option describes independent goods, which do not influence each other’s demand. The third option relates to consumer trends but does not specifically address the relationship between complementary goods. The final option refers to competitive demand, which is about substitute goods that limit each other’s sales rather than working together to meet consumer needs. Therefore, the correct understanding of joint demand pertains to how complementary goods interact in the market.