Final answer:
D. Complementary goods
If the price decrease of good X leads to an increased purchase of good Y, X and Y are complementary goods, resulting from a negative cross-price elasticity.
Step-by-step explanation:
If a decrease in the price of good X causes more units of good Y to be purchased, X and Y are complementary goods. This relationship showcases a negative cross-price elasticity: when the price of one complement decreases, the demand for the associated good tends to increase.
For instance, if the price of coffee (good X) decreases, the consumer may buy more sugar (good Y) because these two are often used together. This is different from substitute goods, where an increase in the price of one leads to a greater quantity consumed of the other.
If a decrease in the price of good X causes more units of good Y to be purchased, X and Y are substitute goods. Substitute goods have positive cross-price elasticities of demand: if good A is a substitute for good B, like coffee and tea, then a higher price for B will mean a greater quantity consumed of A.
Complement goods have negative cross-price elasticities: if good A is a complement for good B, like coffee and sugar, then a higher price for B will mean a decrease in quantity consumed of A.