Final answer:
The scenario where a price decrease in good X leads to less demand for good Y indicates that X and Y are substitute goods, such as coffee and tea. This is characterized by a substitution effect, where consumers favor the cheaper good over its substitute.
Step-by-step explanation:
When the price of good X decreases and this results in fewer units of good Y being demanded, this indicates that X and Y are substitute goods. Substitute goods are products that consumers can use in place of one another. A decrease in the price of one substitute leads consumers to purchase more of it and less of the other product, due to the substitution effect. An example of substitute goods would be coffee and tea; if the price of coffee goes down, people might buy less tea.
On the other hand, if good X and good Y were complementary goods, a decrease in the price of X would result in an increase in the demand for Y. Examples of complementary goods include coffee and sugar. The demand for complements moves in the same direction due to their combined use. The scenario described in the question clearly reflects the nature of substitutes and not complements.