The answer is B. a community interaction.
Let's distinguish some terms:
A population is a group of organisms of the same species in an area. If there were a herd of only zebras, this would be a population interaction.
A community is a group of organisms from different species in an area. A herd of zebra eating grass includes groups of organisms from different species is an example of a community interaction.
An ecosystem consists of all organisms (from different species) in an area and their physical environment.
A biome is a large area defined by abiotic factors.
