difference between b and d??

I would also love clarification between b and d.

me toooo^^^

^^^

^^also having trouble seeing the difference

In this question, we have a couple of "levels," there are groups of mamas species, like rodents or apes, and individual species, like gorillas or rats, and then individual animals, who belong to that species, like Remy the rat or one specific gorilla. B tells us that the various types of rodent species (rat, mouse, capybara) tend to have more individuals in them than do species of other mammals (like humans, dogs, or possums. D, however, tells us that the single species with the most individuals is a rodent species. B talks about the average number of individuals per species, D tells us about the single largest species