The correct answer is option 4, Paramecium. Paramecium is a single-celled organism and belongs to the group of organisms known as protozoans. Protozoans are unicellular eukaryotic organisms that have complex structures and functions within a single cell. They are not considered multicellular because they do not have specialized cells or tissues like multicellular organisms.

Option 1, Fungi, is a multicellular organism. Fungi are eukaryotic organisms that have a complex network of cells called hyphae, which form a mycelium. They have specialized cells and perform various functions in a coordinated manner.

Option 2, Spirogyra, is also a multicellular organism. Spirogyra is a filamentous green algae that consists of multiple cells arranged in a chain-like structure.

Option 3, Hydra, is another multicellular organism. Hydra is a small freshwater polyp that has specialized cells for different functions, including feeding, reproduction, and protection.

Therefore, the correct option is 4, Paramecium, as it is a single-celled organism and not multicellular.