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.

it takes hundreds of years to become stable.

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kelp are large brown algae that live in cool, relatively shallow waters close to the shore. they grow in dense groupings much like a forest on land. these underwater towers of kelp provide food and shelter for thousands of fish, invertebrates, and marine mammal species.

e) all of the above