In order to determine whether a cell is a prokaryote or a eukaryote while looking under a microscope, you can look for specific cellular structures. One key feature that differentiates eukaryotic cells from prokaryotic cells is the presence of a nucleus.
Answer: c) a nucleus
Eukaryotic cells possess a well-defined nucleus where their DNA is enclosed, making it distinct from prokaryotic cells, which lack a nucleus. While both prokaryotes and eukaryotes have a plasma membrane and may have a cell wall (notably in plant cells for eukaryotes), the defining characteristic that confirms it is a eukaryote is the nucleus. Hence, if you see a nucleus in the cell, it indicates the cell is a eukaryote.