Final answer:
The most appropriate variable to show with graduated color is 'Total energy usage in an area,' as it is a continuous, spatially variable data set that lends itself well to color gradients. Option D is the correct answer.
Step-by-step explanation:
The question relates to the most appropriate variable for representing data through graduated color on a map or chart. In a geographical context, graduated color schemes are usually best applied to data that represent continuous quantities or intensities across a geographic area, rather than discrete categories or counts.
Total energy usage in an area is the most suitable variable for representation with graduated color. This is because energy usage is a continuous data variable that can vary across a range of values and is spatially distributed. Representing this information through different shades or intensities of color can help visualize the differences in energy consumption across different areas, which can be related to economic well-being or population density.
In contrast, the other options like the number of high school graduates, number of households under poverty, or crop yield per acre involve discrete counts or measures that may not be as effectively represented with a color gradient.
Thus, the correct option is d) Total energy usage in an area.