When a thin onion peel is stained and viewed under a microscope, it shows a regular arrangement of uniform, brick-like compartments. These compartments are cells, the basic structural and functional units of every living organism. The key teaching point of this classic experiment (first inspired by Robert Hooke's observation of cork) is that an apparently simple onion is in fact built from a large number of similar small units called cells.

Therefore, the correct conclusion is that all onions are made up of similar small structures called cells.

"The size of the onion decides the shape of its individual cells" is wrong because cell shape is determined by the cell's function and its rigid cell wall, not by the overall size of the bulb. "Onion bulbs contain different cell types for different functions" overstates what a single onion-peel slide shows; the peel reveals a uniform population of similar cells, not a variety of specialised types. "Each onion peel contains cells of different composition" is incorrect because the cells seen are essentially alike in structure and composition, which is exactly why they appear as a uniform, repeating pattern.