I would say D, for both B and C.
Taiga biomes often have plenty of trees, so A is obviously no the answer.
Chaparral biomes have very few trees, so B would fit into the answer;
However, grassland biomes are covered mostly by grass. They are prone to fires, preventing too many trees from growing. Due to this knowledge, I can draw the conclusion that the answer B and C will work, meaning the answer is D.