It's all about relativity! For this question, you have to remember that the only components that have mass are the two balls (M1 and M2) and the two cords. M2>M1 and I think we knew that the mass of the cords was small. If we take just the cords into account, the center of mass is going to be somewhere close to b. But we have to take into account the mass of the two balls. The center of mass will lie closer to the heavier ball which is M2. The closest point to M2 is d.
Oh, if you want a formula for center of mass: (m1 x1) + (m2 x2) + (m3 x3) / (m1+m2+m3)