Final answer:
The most important observation in the described experiment is the random selection of sunny and shady test plots to ensure that sunlight exposure does not become a confounding variable and skew the results.
Step-by-step explanation:
Regarding the student's question on experimental procedures and important observations, it seems the topic relates to control variables in a scientific experiment. The most important factor in addressing this inquiry would be the random selection of sunny and shady test plots, as this addresses a potential confounding variable that could influence the growth rate of the plants. If the sun-exposed and shaded pots are not chosen randomly, then the experimental results may be skewed, leading to inaccurate conclusions about the relationship between plant growth and the presence of detergent in the soil. In the provided example where the detergent mix and control seed pots are divided between sunny and shady areas without random assignment, the experiment is flawed as it introduces additional variables (sunlight exposure) that are not being controlled for.