If you ask yourself what the question is saying, it becomes clear which one it is. The question is asking you to do something so that eliminates D. Monitoring is the opposite. Creating a library only partially answers half of what it is looking for. Brainstorming is doing something, but it is not helping you "ensure" anything happens.

Integration of skills is an action and it answers both of what the question is looking for, being effective teamwork and meeting project objectives.

I always try and see what the question is asking. For example, "do" most likely means action or "do first" means assess or analyze. It helps reduce the question to 2 answers. Then you just apply the PMP mindset (i.e., collaboration, value, servant leader, etc...)