What are bacteria classified as?

Bacteria are classified as single-celled organisms that lack a nucleus, which places them in the category of prokaryotes. This classification distinguishes them from eukaryotic organisms, which have cells containing a nucleus and other membrane-bound organelles. The absence of a nucleus means that a bacterium's genetic material is not enclosed within a membrane; instead, it floats freely in the cytoplasm. This fundamental characteristic is a key aspect of what defines bacteria and influences their structure, function, and reproduction.

The other options describe characteristics associated with different types of organisms. Multi-celled organisms with a nucleus include plants, animals, and fungi, which have complex cellular structures. The description of "none-celled parasites" does not accurately fit bacteria, as they are indeed single-celled and can be classified as either beneficial or harmful to other living organisms. Lastly, multi-celled organisms that carry oxygen typically refer to plants or animals with specialized systems for oxygen transport, which again does not apply to the single-celled nature of bacteria. Hence, the correct classification of bacteria as single-celled organisms without a nucleus highlights their unique biological characteristics.