Available choices: (1). solid, liquid, gas. (2). microorganisms. (3). electromagnetic energy. (4). animal behavior. (5). biological diversity. (6). an object. (7). an organism's body. (8). metabolic reaction.

You are presented with the quiz "what do scientists study?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "ethologists usually study how animals behave in their natural environment" and "ethologists are scientists who study animals in their natural habitats". So, what's the best answer to the question?
Answer:
(4).