Which tool is used to measure distance on a Mercator chart?

The divider is the correct tool for measuring distance on a Mercator chart because it allows for accurate and direct measurement between two points on the chart. By placing the tips of the divider on the points of interest, you can easily read off the distance on the scale of the chart. This is particularly important on Mercator charts, which are designed for navigation and provide a consistent scale along lines of latitude.

Using a divider ensures that you can maintain the angle and proportionate size of the distance being measured, which is critical for plotting a course or determining distances when navigating. The other options, while useful in their own contexts, do not provide the same level of accuracy for measuring distances on a chart. A pencil is primarily for marking, a ruler is generally more suited for straight-line measurements on flat surfaces, and a caliper is specific for measuring the dimensions of objects rather than distances on a chart.