To determine whether two goods are substitutes or complements, an economist would estimate the a. price elasticity of demand. b. income elasticity of demand. c. cross-elasticity of demand. d. price elasticity of supply.

Let's briefly review the terms in each option to choose the correct one: a. Price Elasticity of Demand: This measures the responsiveness of the quantity demanded of a good to a change in its price. It is not directly related to understanding the relationship between two goods, so this is not the answer. b. Income Elasticity of Demand: This measures how the quantity demanded of a good changes with respect to changes in consumers' income. Again, it does not directly relate to understanding the relationship between two goods, so this is not the answer. c. Cross-Elasticity of Demand: This measures how the quantity demanded of one good changes as the price of another good changes. This directly relates to understanding if two goods are complements or substitutes, making it our focus for further analysis. d. Price Elasticity of Supply: This measures the responsiveness of the quantity supplied of a good to a change in its price. Like options a and b, this does not directly relate to understanding the relationship between two goods, so this is not the answer.

Since option c, cross-elasticity of demand, directly relates to understanding the relationship between two goods, let's further analyze its definition and implications: - If the cross-elasticity of demand is positive, it means that when the price of one good increases, the quantity demanded of the other good also increases. This indicates that the goods are substitutes. - If the cross-elasticity of demand is negative, it means that when the price of one good increases, the quantity demanded of the other good decreases. This indicates that the goods are complements. - If the cross-elasticity of demand is zero or close to zero, it suggests that the two goods are unrelated in their demand.

Analyzing the given options and definitions, it is clear that an economist would estimate the cross-elasticity of demand (option c) to determine whether two goods are substitutes or complements.