To study the relationship between a supervisor's level of stress and the number of subordinates, a researcher would use:

In studying the relationship between a supervisor's level of stress and the number of subordinates, correlational research is the most applicable method. This type of research aims to determine whether there is a statistical relationship between two variables. In this case, the supervisor's stress level and the number of subordinates can vary independently, and correlational research can effectively measure how these two variables interact.

This method allows researchers to investigate patterns and trends without manipulating any variables, which is important since the researcher is interested in natural variations in stress levels as they relate to management style and the size of the team. By using correlational research, the researcher can collect data, perform statistical analyses, and gain insights into whether an increase in the number of subordinates corresponds to an increase or decrease in stress levels.

Ultimately, this approach will help establish whether there's a relationship worth exploring further, potentially guiding future studies toward more in-depth investigations, like experimental research, which could test causality rather than merely correlation.