Answer:
C. PPC
Step-by-step explanation:
Option A cannot be the answer as anyone can post their desired answers in a forum. More importantly, the forum is not a marketing platform.
Option B is not an answer because, for email, no one has to pay, or it is not an advertising form of bidding.
Option D is wrong because social networking cannot give any user any money until he or she has any approved pages or groups.
Pay per click is an advertising bidding system that pays a user for each click on specific ads. It is also a marketing model. Therefore, PPC is the correct answer.