Answer:
The correct answer will be- complex system.
Step-by-step explanation:
A scientific investigation is performed to understand and explain the natural phenomenon using the scientific method.
An investigation is performed in one of the three ways: descriptive, comparative and experimental.
In the descriptive investigation, the investigation provides a descriptive account of any "complex system" by observing it when no prior knowledge is known. Therefore this method does not include the formation of a hypothesis.
Thus, complex system is the correct answer.