An example of complementary goods would be
a. computers and printers.
b. cars and trucks.
c. sheets and towels.
d. food and clothing.

The correct answer should be A. computers and printers

Complementary goods are those whose values change when the value of one changes. If a computers get bought more due to their price decreasing, printers will also get bought more because you would be able to pay for them since their price would probably decrease too. Other mentioned pairs are not that related to each other.