A) Grassland: Most & Tundra: Least - This answer is incorrect, because grassland biomes are definitely not known for their high plant diversity. As one could imagine, the primary plant found here is grass (and large shrubs.)
B) Rain Forest: Most & Taiga: Least - While this answer is pretty close, a taiga biome does not hold the least amount of plant diversity.
C) Rain Forest: Most & Tundra: Least
D)Temperate Forest: Most & Taiga: Least: As mentioned in B, Taigas do not hold the least amount of plant diversity.
ANSWER:
C) Rain Forest: Most & Tundra: Least