Answer: b.Keystone species
Step-by-step explanation:
A keystone species is the one which plays crucial role in an ecosystem necessary for the proper functioning of the ecosystem. A keystone species makes the environment suitable for the survival of other organisms. Elephant is an example of keystone species. In the given situation the elephants have transformed the woodland into grasslands by removing the trees. This results in the increase in the space for the growth of the grasses. The grasses can be used by the diversity of the herbivorous animals as a source of food who are not capable of grazing leaves from trees.