Final answer:
Demand does not change when the price of a good changes due to the substitution effect and the income effect, which alter the quantity demanded but not demand itself. The correct answer is D. Elasticity of demand.
Step-by-step explanation:
When the price of a good changes, demand does not change because of the substitution effect and the income effect. These two effects describe different but related reactions to price changes. The substitution effect occurs when consumers react to a price increase by consuming less of that good and more of another substitute good, or vice versa when the price falls. The income effect happens when a price change affects the consumer's purchasing power; if the price drops, the consumer can afford to purchase more of the good with the same income, effectively increasing their buying power.
Neither of these effects changes the overall demand for a product; instead, they change the quantity demanded. That's why, when the price of a good changes, we see a movement along the demand curve, not a shift of the demand curve itself. The correct option that explains why demand does not change when the price of a good changes is D. Elasticity of demand.