The correct answer is option 4, Amoeba. Amoebas are classified as single-celled organisms belonging to the group called protozoans. They are known for their ability to change shape and move by extending their pseudopodia, which are temporary extensions of their cytoplasm. Amoebas can be found in various environments including freshwater, saltwater, and soil. They are relatively large for a single-celled organism, with some species reaching sizes of up to 1 millimeter in diameter. This makes them the largest single-celled organism known. Yeast, option 1, is a type of fungus that consists of small single-celled organisms. Acetabularia, option 2, and Acetobacter, option 3, are both multicellular organisms and not single-celled.