I believe it is C. I don't remember the experiment too well, but I know that he is known for discovering lots about genetics and that he was observing genetic traits, so that means it probably is not B or D. I don't even know what A is saying. C makes sense and it almost certainly the answer, as he was testing to find things like recessive traits.
Plants are all of the above (I,II,III) but fungi are heterotrophic, but other than that they have a cell wall in their eukaryotic cells and are multicellular, so it is most likely D.