Answer:
Variable.
Step-by-step explanation:
As it's name suggests, a variable is something (like the exercise indicates could be a situation) that can change or fluctuate over time. This plays a critical role in psychological research given that many changes might occur during a research or an experiment and understanding and finding the variables is key as they can change or fluctuate over time. This would allow a better understanding as to how something evolves into something else.