Final answer:
The answer to the student's question is d) An increase in consumer demand for watermelons, which leads to a rightward shift in the demand curve and results in both an increase in the equilibrium price and quantity of watermelons.
Step-by-step explanation:
The student asked which of the following would cause an increase in the equilibrium price and an increase in the equilibrium quantity of watermelons. The correct answer is d) An increase in consumer demand for watermelons.
When consumer demand for a product increases, this is represented by a rightward shift of the demand curve in a standard supply and demand graph.
As a result, both the equilibrium price and the equilibrium quantity will increase.
Conversely, a decrease in consumer income, a decrease in the price of fertilizer, or an increase in the cost of watermelon seeds does not fit the scenario where both price and quantity increase simultaneously.
A decrease in income usually leads to a decrease in demand, and a decrease in the price of inputs (like fertilizer) generally leads to an increase in supply, which tends to lower the equilibrium price and increase the equilibrium quantity.