Complementary goods refer to:

Complementary goods are defined as products that are used together in such a way that the demand for one good increases when the price of the complementary good decreases. This relationship is based on how these goods enhance each other's utility and satisfaction for consumers. For example, if the price of printers falls, the demand for ink cartridges (a complementary good) is likely to increase because more people are purchasing printers and will need ink to use them.

This concept emphasizes the interdependent nature of consumer choices regarding complementary goods; as one becomes more affordable or attractive, it subsequently drives up demand for its complement. In contrast, the other choices present different economic concepts. For instance, the first option describes substitutes, which are goods that can replace one another rather than complement each other. The third option is too broad and does not accurately capture the specific relationship that defines complementary goods. The last option talks about unrelated goods, which directly contradicts the essence of complementary goods, as these goods inherently interact in consumer usage. Thus, understanding the unique interaction that defines complementary goods underscores why the correct answer articulates this economic relationship accurately.