Choice "C" is correct. If quantity demanded for a product goes up, this drives price up. Additionally, if supply decreases, this will also drive prices up. Therefore, it is a certainty that price will be driven up, given an increase in demand and a decrease in supply.
Choice "a" is incorrect. Increased supply will reduce (not increase) prices, assuming demand remains constant.
Choice "d" is incorrect. Increased demand will increase price, and increased supply will reduce price. The net impact on price cannot be determined without more facts.
Choice "b" is incorrect. Increased demand will increase (not reduce) price, assuming supply remains constant.