답은 on which라고 하는데, 뒤문장이 불안전한데, 어찌 on which가 답이 될수 있나요?

researchers studied two mobile phone companies trying to solve a technological problem. one company developed what it called a “technology shelf,” created by a small group of engineers, (a)[which / on which] were placed possible technical solutions that other teams might use in the future.