Final answer:
The major difference between a single eukaryotic cell organism and a multicellular organism like a human is the number of cells.
Step-by-step explanation:
When comparing a single eukaryotic cell organism with a multicellular organism such as a human, the major difference is the B) Number of cells. A single-celled eukaryotic organism is made up of just one cell, while a multicellular organism like humans consists of trillions of cells. Both types of organisms have cells with a nucleus, hence the presence of a nucleus (C) is not a distinguishing feature between the two. Additionally, both unicellular and multicellular eukaryotes can have a variety of cell wall compositions (A) and modes of reproduction (D), thus these do not represent major distinguishing factors either.