The law of demand states that the quantity demanded of a good changes, other things being equal, when a. the price of the good changes. b. consumer income changes. c. the prices of other goods change. d. a change occurs in the quantities of other goods purchased.

Let's consider each option: a. the price of the good changes: The law of demand shows the relationship between the price of a good and the quantity demanded, assuming that all other factors are held constant. If the price of the good increases, the quantity demanded decreases, and vice versa. This option fits well with the definition of the law of demand. b. consumer income changes: While consumer income can affect the demand for the good, it is not the primary focus of the law of demand as it assumes all other factors are held constant. So this option doesn't fit the definition. c. the prices of other goods change: The prices of other goods can have an impact on the demand for a particular good through substitution or complementary effects, but these factors are not the primary focus of the law of demand. Therefore, this option doesn't fit the definition. d. a change occurs in the quantities of other goods purchased: This option considers the changes in the quantities of other goods purchased, which may have an effect on the demand for the good in question, but it is not the primary focus of the law of demand as it assumes all other factors are held constant. So this option doesn't fit the definition.

Based on our analysis, the best answer that describes the law of demand is: a. the price of the good changes. This option focuses on the relationship between the price of a good and the quantity demanded, which is the key aspect of the law of demand. It states that the quantity demanded of a good changes when the price of the good changes, keeping other factors constant.