How would a scientist have made the distinction between a unicellular and a multicellular organism in the 17th century (1600s), the mid-20th century (1900s), and in the 21st century

Antony Van Leeuwenhoek invented the first microscope that could see cells. So, in the 17th century, a simple microscope must have been used to see the cells and distinguish between them.

The 20th century saw a development in the field of microscopy. Microscopes like phase contrast microscope developed by Frits Zernike and transmission electron microscope invented by Ernst Ruska and Max Knoll  were used for many purposes.

In the 21st century, microscopes like digital and stereo microscopes are used.

there are mainly three domains in biological classification namely; archaea, bacteria, and eukarya.

archaea includes all the prokaryotic cells which are characterized by their lipid membranes. they have branched hydrocarbon which is attached to glycerol through an ether linkage.

bacteria includes all the prokaryotic cells, the cell membrane of which are made up of the unbranched fatty acid chain. in addition, they are attached to glycerol through an ester bond.

eukarya includes all the eukaryotic cells, that is, the cells contain a nucleus and other membrane-bound cell organelles.

they can be autotrophic as well as heterotrophic, unicellular as well as multicellular

they include kingdoms like protista, plantae, animalia, fungi et cetera.

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