Which type of chart is used to compare parts of a whole?

A pie chart is specifically designed to show how different parts contribute to a whole. Each slice of the pie represents a category's proportion relative to the entire set of data. This visualization effectively communicates the relative sizes of segments in a clear and intuitive manner, making it easy to see which categories are larger or smaller.

In contrast, column charts display values vertically and are better suited for comparing different categories or groups across a common scale, rather than emphasizing their relation to a whole. Line charts are used to illustrate trends over time, connecting data points to show changes, which does not convey the concept of parts of a whole. Bubble charts incorporate a third dimension through the use of bubble size, emphasizing the relationships among three variables, and aren't focused on comparing parts of a whole either. Thus, the pie chart stands out as the most appropriate choice for the specified purpose.