I believe the answer is: C) is lost from the system as heat energy.
Plants had the largest amount of energy in the food web.
When the herbivore consume the plant, the energy from the plant would be transferred to the herbivore.
After that, this energy would be burned out through metabolism as heat energy when the herbivore does its daily activity.
This is why when carnivore consume the herbivore, they would obtain less energy.
