The correct answer is option 4 - Hydra. Hydra is a multicellular organism, not a unicellular organism. Hydra belongs to the phylum Cnidaria and is classified as a simple aquatic animal. It has a tubular body with tentacles and is found in freshwater environments. Unlike unicellular organisms, Hydra is composed of multiple cells that work together to perform various functions such as digestion, reproduction, and movement. This makes Hydra different from options 1, 2, and 3 - Euglena, Amoeba, and Paramecium, which are all unicellular organisms. Unicellular organisms are made up of a single cell and can carry out all necessary life functions within that one cell.