Which type of chart is designed to show the relationship between two quantitative variables?

The choice of a scatter diagram is ideal for illustrating the relationship between two quantitative variables because it allows individual data points to be plotted on a two-dimensional graph. Each axis represents one of the quantitative variables, which enables viewers to visualize how changes in one variable correspond to changes in the other. This visual representation can reveal patterns, correlations, or trends within the data set, making it a powerful tool for analysis.

In contrast, pie charts are typically used to display proportions of a whole, bar graphs are effective for comparing discrete categories, and line graphs illustrate trends over time rather than directly showing the relationship between two quantitative variables. Therefore, the scatter diagram stands out as the best choice for analyzing relationships between two quantitative factors.