A and B eliminated because visualization and coordination are not sufficient necessary relationship (one doesn't need to happen for the other to exisit).

C eliminated because the opening of second sentence tells us "if the sliding doors are open, as it MAY happen during summer," this doesn't mean the doors have to always be open during summer in order for the garden and living room to merge. In fact, the first sentence tells us ANY garden and living room can merge (closed or open). It just that the effect and intesification that makes it a difference when the door is open.

E eliminate (I had a hard time with this one). But the problem here is that it's implying cold month situation but we don't really know about doors open during cold months so that's why I didn't like this one. It also suggesting that only summer can intesify but we don't know if that's a must be true (you need to approach soft much be true same as must be true. I know some companies say differently but that's how I learned it).

D correct because the first sentence tells us any garden can be merged with living room. We just know the effect remains strong during cold months if the coordination is there.