The correct answer is A. rises.
When the price of a substitute of commodity X falls, the demand for X will rise. This is because consumers will now be able to get the same satisfaction from consuming less of X and more of the substitute. This is known as the substitution effect.
In addition, the fall in the price of the substitute will make X relatively more expensive. This is known as the income effect. The income effect will also lead to an increase in the demand for X.
Therefore, the demand for X will rise when the price of a substitute of commodity X falls.
Option B is incorrect because the demand for X will rise, not fall.
Option C is incorrect because the demand for X will change, not remain unchanged.
Option D is incorrect because only option A is correct.