Final answer:
An increase in price would directly lead to a decrease in quantity demanded.
Step-by-step explanation:
Of the options given, an increase in price would directly lead to a decrease in quantity demanded. This is because as the price of a product increases, consumers are less willing or able to purchase the product, resulting in a decrease in demand.
An increase in consumer income would actually lead to an increase in quantity demanded. When consumers have more income, they are more likely to purchase goods and services, leading to an increase in demand.
An increase in the price of a related good could also lead to a decrease in quantity demanded. For example, if the price of a substitute good increases, consumers may choose to purchase the original good instead, resulting in a decrease in demand for the substitute good.
Therefore, the correct answer is A. increase in price.