A small living plant or animal that cannot be seen without the aid of a microscope is option (d.) Micro-organism.
It is an organism that can be seen only through a microscope, and not with eyes. Microorganisms consists of bacteria, protozoa, algae, and fungi. However, the viruses aren't considered as living organisms, but are often classified as microorganisms.
Micro-organism/microbe is just like any living organism that spends its life in such a tiny size which is hardly possible to see without the microscope.
They play a major role in human welfare and the environment, such as processing and preservation of food,
The microorganisms are involved in the production of oxygen, biomass control and cleaning the Earth of remnants of dead organisms.
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