Question

Rabbits are introduced into a grassland area. Which action of the rabbits would harm the buffalo that live in the grassland?

Answer 1

The most likely answer choice should and would be B, given that rabbits are herbivores (meaning that they only eat vegetation and plants, including grass). Buffalo's food source(s) and diet consists of grass as well. If enough rabbits are introduced into a grassland area, they will furthermore consume more grass, in turn limiting the food source of the buffalo. 

''Attacking'' prey doesn't necessarily make sense, given that buffalos do not 'prey' on any animal and rabbits themselves are prey.

A to a lesser extent can make sense, although I doubtable.  B's most likely the answer.