Final answer:
A microbe is defined as a single-celled organism that can perform the basic functions of life, usually too small to see without a microscope, and can exist as unicellular or multicellular (and even acellular) forms.
Step-by-step explanation:
The correct definition of a microbe is b. single-celled organisms that can perform the basic functions of life.
Microbes, also known as microorganisms, are living entities that are typically too small to be seen without the aid of a microscope.
They vary greatly in size, structure, habitat, and metabolism. While many are unicellular, such as bacteria and archaea, there are also multicellular organisms that fit into this category.
Some microbes, like viruses, don't even have cellular structures, as they are acellular.
A cell is the smallest independently functioning unit of a living organism, and the unified cell theory states that all organisms are composed of one or more cells, with the cell being the basic unit of life.
Cells are the basic building blocks of all living organisms, from single-celled bacteria to multicellular human beings.