Final answer:
The correct answer is option c) Operational definition.
Step-by-step explanation:
In Robert Levine's research on the "pace of life" in 31 different countries, the measure of the amount of time it took a pedestrian to walk 60 feet on a downtown city street is an example of an operational definition. This is because it is a specific procedure used by the researcher to quantify the concept he is interested in measuring - the pace of life. Therefore, the correct answer to the student's question is c) Operational definition.
In any experimental design, like Levine's, the choice of what and how to measure variables is crucial for the validity of the results. In this case, the pace at which pedestrians walk is expected to reflect the overall pace of life in different cities and countries, tying a concrete, measurable action to an abstract concept.