In descriptive scientific investigations, scientists often make observations to understand the interacting parts of a complex (blank)

The correct answer will be complex system.

Explanation:

A descriptive scientific investigation is an investigation which describes and quantify the natural phenomenon.  

The scientists use this method when very little is known about the natural system sue to which fewer predictions are made. Thus, no hypothesis is proposed in this type of investigation.  For example- to see an unknown microscopic organism.  

Thus, a complex system is the correct answer.