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The PAS (Periodic Acid-Schiff) stain is particularly effective at highlighting glycogen-rich structures in histological sections because it reacts specifically with polysaccharides, including glycogen, and certain glycoproteins. The staining process breaks down polysaccharides to produce reactive aldehyde groups that can be visualized with the Schiff reagent, resulting in a bright magenta color under the microscope.

Glycogen, which is a major storage form of carbohydrates in the body, is found primarily in the liver and muscle tissues. Therefore, when a tissue sample is treated with the PAS stain, any areas containing glycogen will appear distinctly colored, allowing for easy identification and assessment of glycogen storage disorders or the function of these organs.

While mitochondria, ribosomes, and nuclei are vital organelles that perform essential cellular functions, they do not have the specific chemical characteristics required to be visualized by the PAS stain. Thus, the primary purpose of PAS staining is to visualize glycogen and other carbohydrate-rich structures, making glycogen-rich structures the correct answer to the question.