A way online advertising can not be purchased is
D.Cost per action
Step-by-step explanation:
Cost per action means that the company will pay to the people who are giving them ad space only when the desired action they want done from the ad is actually taken.
This defeats the main motive of advertising and makes it possible for there to be malpractices involved.
Advertising is to get a product be seen by people and the advertiser has no obligation to make people buy the product they are advertising.
This is why cost per action is not permissible and against the guidelines.