Which of the following chart types is used to demonstrate relationships between variables?

The bubble chart is specifically designed to illustrate relationships between multiple variables, making it a powerful tool for visualizing data sets where the interaction between these variables is crucial. In a bubble chart, each data point is represented as a bubble, and the position of the bubble on the x and y axes signifies the values of two of the variables. Additionally, the size of the bubble can represent a third variable, allowing for a multifaceted view of the data.

This capability to encapsulate relationships among variables is what distinguishes the bubble chart from other chart types. For instance, a pie chart is meant for showing proportions of a whole, a line chart is primarily used to depict trends over time, and a column chart is best suited for comparing categorical data across different groups. Each of these other chart types focuses on different aspects of data visualization, whereas the bubble chart excels at demonstrating how multiple variables interrelate, thus making it the most suitable choice for the question posed.