The right answer to this question is option B. Plankton
What you need to understand to answer this question is that, almost always, we'll have plants at the bottom of all food webs, want to know why? Well, it's because of them that all animals have food to eat, even though an animal only eats meat, the animal he ate meat from, probably ate vegetables to do so, that's why plants are almost always in the bottom and are the foundation of the food web.
But in this case, why is Plankton in the bottom? That's a good question, it is because they are the main source of energy of many animals on the ocean for example, just like the plants on the ground, that's why they are in the bottom.
