Which of the following causes a downward movement along the demand curve :

Fall in the price of the commodity

The correct answer is option (3) : Fall in the price of the commodity

Fall in the price of the commodity: When the price of a commodity decreases, all else being equal, consumers tend to buy more of that commodity. This results in a movement down along the demand curve, showing an increase in quantity demanded.

Let's analyze the other options:

• Rise in the income of the consumer: An increase in consumer income typically shifts the entire demand curve to the right (for normal goods) or left (for inferior goods), rather than causing a movement along the curve.

• Fall in the price of substitute goods: A decrease in the price of substitute goods would shift the demand for the original commodity to the left (decrease demand) or right (increase demand), not cause a movement along the existing demand curve.

• Rise in price of substitute goods: An increase in the price of substitute goods would generally shift the demand for the original commodity to the right (increase demand) or left (decrease demand), rather than cause a movement along the demand curve.