Final answer:
The most useful BI tool for determining the relationship between population sizes by ZIP code and crime rates, and for visualizing this relationship in a graph, would be statistical analysis software, which can perform statistical tests and create visual data representations.
Step-by-step explanation:
To determine if there is a relationship between population sizes by ZIP code and crime rates, and to display the results in a graph, statistical analysis software would be the most useful BI tool. This type of software is capable of conducting various statistical tests, such as correlation or regression analysis, to identify if there is a statistically significant relationship between the two variables.
Statistical software can also generate graphs and visual representations like scatter plots to help visualize the data. Moreover, if the data has a geographical component, using GIS software could be beneficial for spatial analysis and mapping patterns, such as hotspot analysis or nearest neighbor analysis.