Final answer:
An increase in the demand for a good means that consumers are willing to purchase more of the good at each possible price, represented by a rightward shift of the demand curve. The correct answer is option 4.
Step-by-step explanation:
An increase in the demand for a good implies that consumers are now willing to purchase more of the good at each possible price. This situation is represented graphically as a shift of the demand curve to the right. It is not a simple movement along the demand curve, which would imply a change in the quantity demanded due to a change in the good's own price. Instead, this shift can be due to a variety of factors such as an increase in consumer income, an increase in the number of consumers, an increase in tastes or preferences, an expectation of future price increases, or an increase in demand for a complementary good.
Regarding our options listed, option 1) is incorrect because an increase in demand specifically causes a shift to the right as people are willing to buy more at each price level. Option 2) is also incorrect because the increase in the quantity demanded as a result of a fall in the price of the good is a movement along the demand curve, not a shift in demand. Option 3), which states the good has become scarce, does not accurately explain a rightward shift in the demand curve as scarcity is a supply-side factor.
Therefore, option 4) is the correct answer because it accurately describes that consumers are willing to purchase more of the good at each possible price, which means the demand curve has shifted to the right.