Cells are the basic structural and functional units of life. this statement is
considered a scientific theory because

a different kinds of cells have different structures and functions
b. every living thing observed so far consists of one or more cells.

c. the structure of a skin cell enables the cell to perform all of its
functions

d. all cells are able to exchange matter and energy with their
surroundings

The scientific theory of cells, also referred ‘Unified Cell Theory’ or ‘Unifying Principle of Biology‘ was proposed by scientists like Theodor Schwann, Matthias Schleiden, and Rudolph Virchow.

Explanation:

The basic principles of this theory are:

An organism is unicellular if it has a single cell like the amoeba, paramecium etc. An organism is multicellular if it has multiple cells like birds, reptiles, human beings, etc. A single cell can be simple or complex but any living organism is essentially composed of at least a single cell.

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