Organisms lacking a well-defined nucleus are termed as —

✅Explanation
-Prokaryotes are single-celled organisms characterized by the absence of a true nucleus and other membrane-bound organelles. Their genetic material (DNA) is located in a region called the nucleoid, which is not enclosed by a membrane. Bacteria and archaea are the two main groups of prokaryotes.

📌Other Options Explanations:
-A. Fungi: Fungi are eukaryotic organisms, meaning they possess a well-defined nucleus and other membrane-bound organelles. They can be unicellular (yeasts) or multicellular (molds and mushrooms).
-B. Protozoa: Protozoa are also eukaryotic organisms, primarily unicellular, and exhibit a well-defined nucleus and cellular organelles.
-C. Eukaryotes: Eukaryotes are organisms with cells containing a true nucleus and other membrane-bound organelles. This group includes animals, plants, fungi, and protists.