The organisms listed belong to different animal phyla, representing different stages of evolutionary complexity:

B. Paramecium is a unicellular protist, the least complex.

D. Hydra is a simple multicellular organism belonging to the phylum Cnidaria, with only two primary tissue layers and radial symmetry.

A. Planaria is a flatworm (phylum Platyhelminthes), which is more advanced than hydra as it has three tissue layers (triploblastic), bilateral symmetry, and a centralized nervous system (cephalization).

C. Earthworm belongs to the phylum Annelida. Annelids are more evolutionarily advanced than flatworms because they have a true coelom (body cavity), a closed circulatory system, a segmented body plan, and more complex organ systems.

Based on the analysis above, the correct answer is earthworm - option C