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

Complex  System

Explanation:

Given that, a descriptive scientific investigation is one of the three main types of investigation which formulates and quantify the natural phenomenon.  This natural phenomenon oftentimes involves Complex System, such as microscopic organisms, thereby, scientists often make observations to understand the interacting parts of this COMPLEX SYSTEM

Hence, the right answer is a COMPLEX SYSTEM

the leaf is an organ of a typical plant, and the main functions are to complete photosynthesis and allow the plant to exchange gas, just like how we have the respiratory system. leaves have the stomata, which collects maintains co2 and o2 levels. this is necessary for cell metabolism. the leaf also collects sunlight using chloroplasts. succinctly, leaves own a pivotal role in sustaining plant life.