Final answer:
A thermometer would be the most helpful tool for a nurse investigating the effects of environmental factors on patients' body temperatures.
Step-by-step explanation:
The most helpful tool for a nurse investigating how different environmental factors affect her patients' body temperatures would be a thermometer (option C).
Thermometers are specifically designed to measure temperature and are commonly used in healthcare settings to monitor body temperature. They provide an accurate measurement of the patient's body temperature, allowing the nurse to track changes and identify any deviations from the normal range.
For example, modern thermometers placed in the ear canal use infrared technology to measure body temperature quickly and accurately. They are more reliable than older methods such as alcohol thermometers placed under the tongue or in the armpit.