Final answer:
Spirochetes are best visualized using a dark field microscope which allows for high-contrast, live observation of thin, unstained organisms such as Treponema pallidum.
Step-by-step explanation:
The organism that is best visualized using a dark field microscope is C) Spirochetes. Darkfield microscopy is particularly useful for viewing transparent and unstained specimens that are difficult to see under standard light microscopes, such as brightfield microscopes. Spirochetes, like Treponema pallidum, which causes syphilis, are thin and difficult to see with brightfield microscopy. However, they can be easily visualized as bright objects against a dark background using darkfield microscopy. This contrast enhancement allows for the detailed observation of live, unstained spirochetes.
By using a darkfield microscope, we avoid the need for staining which can kill or alter the specimen, making this technique excellent for observing the morphology and motility of live organisms.