* Paramecium: These are single-celled organisms (protists). Their internal structures, like cilia, contractile vacuoles, and the nucleus, are all clearly visible under a microscope. They are small enough to be easily observed with a standard light microscope.

* Earthworm: While earthworms have interesting internal structures, they are much larger than paramecium. You can see some external features, but to see internal details (like the digestive system), you would need a more powerful microscope or dissection.

* Grasshopper: Grasshoppers are complex insects with many intricate structures. However, they are too large for a standard microscope. You'd need a dissecting microscope or specialized equipment to view their internal anatomy.

In summary: Paramecium is the best choice for viewing under a microscope due to its small size and the abundance of easily observable internal features.