Answer: The correct answer is "c. decrease in demand".
Explanation: Complementary goods are all those products that depend on each other. That is, they are so closely linked that the behavior of one inevitably affects the behavior of the other.
The classic example of complementary goods is that of cars and gasoline. The sale of the former may be affected by an increase in the price of the latter; and, at the same time, the consumption of the second depends on the sale of the first.