The amoeba and the paramecia is a single cell organism. The elodea plant cell is a multicellular cell. You can tell if a cell is a single cell by for example the paramecia is single because it only have 1 cell. It does not connect with other cells to be able to live on it's own. Same with the amoeba, it's a single cell because it does not connect to other amoeba to be able to live. The elodea plant cell is a multicellular cell because it is connected to another elodea cells and form a rectangular shape. They need each other to be able to survive. both single cell and multicellular cell are both organisms because it's a living thing.