Final answer:
The statement "polluted water kills fish" made by researchers after observing that fish live in clean water but not in polluted water is an example of a hypothesis. It is a proposed explanation that is testable and based on observations.
Step-by-step explanation:
After observing that fish live in clean water but not in polluted water and stating that "polluted water kills fish," researchers are making a hypothesis. A hypothesis is a proposed explanation based on observations that is testable and can lead to further investigation. While an observation is the act of noting and recording something, such as fish only living in clean water, a hypothesis goes a step further to propose a reason for this observation. The statement by the researchers seeks to explain the observation and can be tested by designing an experiment or observational study to determine the validity of the proposed relationship between pollution and the survival of fish.