WHY IS THE ANSWER D AND NOT B?

Hi, there is no reason that G H and L cannot be on both committees. We are trying to maximize the number of people who are on both committees, so if we maximize that, then those three, the only ones on the planting committee, will have to serve on both committees.

But to clarify why D is right, M must be in, and M cannot be on planting, so trails must have GHLM at least.