Final answer:
A phase-contrast microscope is the best choice for viewing the internal structures of a live protist like Paramecium because it enhances contrast without staining, which allows for the observation of living cells.
Step-by-step explanation:
The best choice for viewing internal structures of a living protist, such as a Paramecium, would be a phase-contrast microscope. This type of microscope is particularly useful for enhancing the contrast in transparent and colorless specimens without the need for staining. Unlike brightfield microscopy, which often requires staining procedures that can kill the cells, phase-contrast allows for the observation of live specimens, thereby enabling one to study the cellular processes in their natural state.
The best choice for viewing internal structures of a living protist such as a Paramecium would be a brightfield microscope with a stain. Brightfield microscopes use visible light to illuminate the specimen, and stains can provide contrast to distinguish the internal structures of the protist. However, it is important to note that staining usually kills the cells being observed.