It's neither. Here 'room' means 'space'.

It means "enough space." Here, "room" is being used as a "mass noun"--I think that's the word--like "air" or "water." "Can all five of us fit into your car?" "Oh, sure, it's a big one--there's plenty of room." "The table was a large one, but the three were all crowded together at one corner of it: `No room! No room!' they cried out when they saw Alice coming. `There's plenty of room!' said Alice indignantly, and she sat down in a large arm-chair at one end of the table."--"Alice's Adventures in Wonderland" When Walt Disney built Disneyland, it was constrained by space. When they built Walt DIsney World, they bought a huge amount of land so that there would be plenty of room for expansion.

The answer is "B". 在设个情况里，"room"是“空间”的意思。所以，"plenty of room" 跟 “plenty of space (空间）是一样。如果有一个房间有很多空间，那个房间就很大。所以，很大的房间 (a large room, answer "B"). 加油！ :)