Answer:
both cooperative learning and problem-centered instruction
Step-by-step explanation:
The collaborative learning method refers to the process of learning in which the group of students involves a particular problem to solve it or to accomplish any specific task. It is believed that learning is a process involving continuous involvement and is considered a social act altogether. In the given case, Mr. WIllis has used both cooperative learning and problem-centered instruction. By separating the students and then collaborating to a conclusion, he used the method of cooperative learning. At the same time, he gave the instruction centered on the problem.