The correct answer is C) "pressure".
Pressure is a technique where the persuader uses intimidation techniques, such as the coercion or threats in order to get support from the target individual. This is a technique normally used in police interrogations where an individual is reluctant to cooperate. Interrogators use this tactic in order to force cooperation from the individual.
As the question mentions it, it always more effective make the target offer you his support voluntarily rather than forcing him to.