Final answer:
Microscopic single-called organisms that range in length from 1 to 20 micrometers are bacteria, which are prokaryotic cells without a nucleus and can be of various sizes, generally smaller than most multicellular organisms and larger than viruses.
Step-by-step explanation:
Microscopic single-called organisms that range in length from 1 to 20 micrometers are called bacteria. The term prokaryotes is often used to refer to unicellular organisms that have no nucleus, which includes bacteria and archaea. Most bacteria measure 1 to 2 micrometers in length, although some prokaryotic cells can be as small as 0.5 micrometers or as large as 500 micrometers. By contrast, viruses are much smaller and simpler organisms, usually ranging from 20 to 300 nanometers and require living cells to multiply. They are not considered alive on their own because they cannot replicate without a host. Unlike bacteria and viruses, fungi can be either unicellular or multicellular and are usually larger than bacteria and viruses. Exotoxins are not organisms at all, but rather toxic substances produced by certain bacteria.