A is not usually thought of as an economic activity, although it could lead to one. Good contacts are never to be dismissed. I'd say A is not the answer.
B helping with decorations. No money or services have exchanged hands. B is not the answer.
D picking up a relative at the airport. Generally D is not the answer. We don't charge our relatives for services given (although if it kept them away, maybe that wouldn't be such a bad thing).
C The key word is selling. Money for products. This is true economics. Here's your answer.