in descriptive scientific investigations scientis often make observations to understand the interacting parts of a complex

The answer is System.

When a scientific investigation is in progress, its used to perform and understand the natural phenomenon that goes on using the scientific method. Since this is a descriptive investigation, it provides a descriptive account of any system by looking at it without any prior knowledge to that subject. This method of investigation does not use a hypothesis. Thus, proves our answer correct.

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