The correct answer is: C. a downward shift in demand for the other commodity.
Complementary goods are goods that are used together. For example, pens and paper are complementary goods. If the price of pens goes up, people will buy less pens. This will also lead to a decrease in the demand for paper, because people will not need as much paper if they are not using as many pens.
A rise in the price of one good will lead to a decrease in the demand for the other good, because the two goods are complementary. This is known as the “complementary goods effect.”
Option A is incorrect because a rise in the price of one good will lead to a decrease in the demand for the other good, not an increase.
Option B is incorrect because a rise in the price of one good will not lead to a rise in the price of the other good.
Option D is incorrect because a rise in the price of one good will lead to a decrease in the demand for the other good, not no shift in demand.