Step-by-step explanation:
viruses viruses might be microcells but they are definitely not the smallest cells
not all cells have a nuclei eg bacteria la la a nucleus
all living things are indeed made of one or more cells that's why we can classify them into either eukaryotes or prokaryotes
D is wrong because we don't classify cells into eukaryotes and prokaryotes but we classify organisms into eukaryotes and prokaryotes