The correct answer is option 3, Amoeba. Amoeba is a single-celled organism that belongs to the group of organisms called protozoans. Protozoans are simple organisms that do not have a complex nervous system like animals. They do not have specialized nerve cells or a centralized nervous system. Instead, amoebas rely on simple reactions to stimuli, such as moving away from harmful substances or towards food sources.

Options 1, 2, and 4, on the other hand, are all animals that do have a nervous system. Leeches, tapeworms, and snails are all multicellular organisms that possess specialized nerve cells and some level of nervous system organization. They are capable of more complex behavior and responses to stimuli compared to amoebas.