Based on the description of the scatterplot titled "Visitors to the Food Truck," the statistical investigative question that can be inferred is related to the age distribution of the visitors. Among the provided options, the question that fits best is:
What percentage of customers is over 21? The answer is 60 percent.
This question directly relates to the x-axis (ages), allowing for analysis of the age groups among the food truck visitors. The other options do not seem relevant to the scatterplot data presented.