The correct answer is A. cross price elasticity of demand.
The cross-price elasticity of demand is a measure of how the demand for a good responds to a change in the price of another good. It is calculated as the percentage change in the quantity demanded of good $x$ divided by the percentage change in the price of good $y$.
A positive cross-price elasticity of demand indicates that the two goods are substitutes. This means that when the price of good $y$ increases, consumers will demand more of good $x$, since it is now a relatively cheaper alternative.
A negative cross-price elasticity of demand indicates that the two goods are complements. This means that when the price of good $y$ increases, consumers will demand less of good $x$, since it is now a relatively more expensive good to consume with good $y$.
B. income elasticity of demand is a measure of how the demand for a good responds to a change in income. It is calculated as the percentage change in the quantity demanded of good $x$ divided by the percentage change in income.
A positive income elasticity of demand indicates that the good is a normal good. This means that when income increases, consumers will demand more of the good.
A negative income elasticity of demand indicates that the good is an inferior good. This means that when income increases, consumers will demand less of the good.
C. price elasticity of supply is a measure of how the quantity supplied of a good responds to a change in the price of the good. It is calculated as the percentage change in the quantity supplied of good $x$ divided by the percentage change in the price of good $x$.
A positive price elasticity of supply indicates that the good is a relatively elastic good. This means that a small change in the price of the good will lead to a large change in the quantity supplied.
A negative price elasticity of supply indicates that the good is a relatively inelastic good. This means that a large change in the price of the good will lead to a small change in the quantity supplied.
D. cost elasticity of supply is a measure of how the quantity supplied of a good responds to a change in the cost of production. It is calculated as the percentage change in the quantity supplied of good $x$ divided by the percentage change in the cost of production of good $x$.
A positive cost elasticity of supply indicates that the good is a relatively elastic good. This means that a small change in the cost of production will lead to a large change in the quantity supplied.
A negative cost elasticity of supply indicates that the good is a relatively inelastic good. This means that a large change in the cost of production will lead to a small change in the quantity supplied.