Answer: a. Differential reinforcement of low rates of behavior
Explanation: Differential Reinforcement is a method that aims to exchange undesirable behavior with appropriate behavior. It reinforces the desired behavior just as the teacher here promises the students bonus point for a reduced rate of the undesirable behavior.
If she reinforces a different behavior, for instance, promising bonus points for directing their questions to the members of other groups, this would be a Differential reinforcement of other behaviors. If she promises bonus points for the most quiet team, this would be a Differential reinforcement of incompatible behaviors as it is impossible to be quiet and ask questions at the same time.