Choice "C" is correct. The fundamental law of demand holds that there is an inverse relationship between price of the product and the quantity demanded. We move along the demand curve D-D.Choice "b" is incorrect. An increase in complementary product prices would decrease the demand curve (e.g., if PC prices increase, the demand for printers and other peripherals decrease).
Choice "d" is incorrect. Increases in consumers and consumer income shift the demand curve itself.
Choice "a" is incorrect. A decrease in price for a substitute product (like Pepsi) decreases demand for the other product (Coke).
