Which type of plot is primarily used to determine the relationship or correlation between two variables?

A scatter plot is specifically designed to display the relationship or correlation between two variables. By plotting individual data points on a two-dimensional graph, where one variable is on the x-axis and the other on the y-axis, you can visually see how the two variables interact with each other. This makes it easy to identify trends, patterns, or potential correlations, such as positive, negative, or no correlation at all.

In contrast, a bar plot is used to compare categorical data, where bars represent different categories rather than relationships between two numerical variables. A pie chart visualizes proportions of a whole, allowing for comparisons of parts of a single categorical variable but lacking the capability to depict relationships between two variables. A histogram is focused on the distribution of a single quantitative variable by displaying the frequency of different ranges of values, rather than examining the relationship between two distinct variables. Thus, the scatter plot is the most appropriate choice for assessing the correlation between two data sets.