I dont want to mislead you, but I believe the answer is A because letter B is wrong because plants go through cellular respiration and photosynthesis, whereas animals go through cellular respiration but not photosynthesis, and for C, I've never heard of that so it doesnt seem likely.
The answer to this is C because humans don't have cell walls and if you are looking at an onion it has a cell wall because it is a plant.
Answer down below:
If a deer population starts eating all the plants in a specific area, the plants, of course will kill the plants. But also, once all the plants will run out, all the organisms and deer will eventually die out.
Awnser is d hope this helps