The answer is: A. Fixed Ratio
When reinforcement is set in a fixed ratio, the researchers will already determine how fast a certain type of punishments or reward would be given to the subjects after they are completing a certain type of behavior.
Example of the experiment would be an experiment where the researchers would provide an exact amount of candies for children as soon as they completed the puzzles that are assigned to them (in order to see whether they are more motivated to complete the puzzle or not)