Final answer:
The correct question is c) What is the turnover time of the material in the reservoir? Turnover time is equivalent to residence time, which reflects the average duration a molecule stays in a reservoir.
Step-by-step explanation:
The correct question that matches the answer “The average time spent by a molecule of the material of interest in a reservoir” is c) What is the turnover time of the material in the reservoir? This is because the term 'turnover time' is synonymous with 'residence time,' which is a measure of the average time an individual water molecule stays in a particular reservoir. In environmental science and biology, this concept is often used to describe water cycling and the movement of other materials within ecosystems. Residence time is important for understanding how long resources are contained within a given reservoir before being cycled back into the ecosystem.