Answer: Option(e) is correct answer
Explanation:
According to the question Helen should reward each student when they contribute in the activities of class through their participation. This will reinforce their act of participation to attain reward.Thus, she will achieve desired results for her class.
Other options are incorrect because making model, yelling at students ,calling on students for participation and giving chances to other students to participate will not invoke reinforcement in students to participant and get desired goals.Thus, the correct option is option(e)