Final answer:
A movement down along a demand curve is caused by a fall in the price of the product, which results in an increase in quantity demanded due to the substitution effect and income effect.
Step-by-step explanation:
The movement down along a demand curve for a product generally indicates that there is an increase in quantity demanded due to a decrease in the price of the product. In the context of the options provided, the correct answer is B. a fall in the price of the product. In such a case, the substitution effect and the income effect both contribute to the increase in quantity demanded. The substitution effect occurs because the product is now cheaper compared to other goods, leading consumers to buy more of it. The income effect is the increase in the purchasing power of consumers, as they can now buy the same amount they used to while spending less money, or they can buy more with the same budget. Other changes such as a change in income, price of substitute goods, or the price of complementary goods, would cause a shift in the demand curve itself, not a movement along it.