Option 1: Fall in price of the given commodity -> Causes downward movement along the demand curve, leading to higher quantity demanded.
Option 2: Rise in the price of substitute goods -> This shifts the demand curve rightward, not a movement along the curve.
Option 3: Rise in price of the given commodity -> This causes upward movement along the demand curve, not downward.
Option 4: Fall in the price of substitute goods -> This shifts the demand curve leftward, not a movement along the curve.
Hence, Option 1: Fall in price of the given commodity -> Movement along the demand curve occurs only when the price of the commodity itself changes. A downward movement means moving from a higher price point to a lower price point on the same curve, which happens when price falls. This results in an increase in quantity demanded according to the law of demand. Changes in prices of substitute goods cause shifts of the entire demand curve, not movements along it. -> correct