Final answer:
To confirm that the cells depicted in Leeuwenhoek's drawing are eukaryotic yeast cells, the drawing must include a defined nucleus, which is a key feature of eukaryotic cells.
Step-by-step explanation:
The element missing from Antonie van Leeuwenhoek's drawing that would confirm that the cells depicted are eukaryotic yeast cells is A. a defined nucleus. Yeast cells are a type of eukaryote, and a characteristic feature of eukaryotic cells is the presence of a nucleus that is surrounded by a nuclear envelope with nuclear pores. This well-defined nucleus distinguishes eukaryotic cells from prokaryotic cells, which lack a nucleus. Eukaryotic cells also have other membrane-bound organelles and a structural network called the cytoskeleton.