Which graphical tool is used to monitor process variables over time to identify trends and cycles?

The correct choice is indeed the Run Chart, as it serves a specific purpose in analyzing how process variables change over time. A Run Chart visually represents data points plotted in the order they occur, which helps in identifying trends, patterns, and cycles within a given timeframe. By tracking performance or other variables over intervals, it becomes easier to discern whether a process is stable, improving, or deteriorating.

In contrast, the other options do not fulfill this role. A Pareto Chart focuses on identifying the most significant factors contributing to a problem, using the principle that a small number of causes often lead to the majority of the problems. A Pie Chart provides a visual representation of the proportions of different parts to a whole, which is not time-focused. Finally, a Histogram is utilized to depict the distribution of data points across defined intervals or bins but does not track how those values change over time. Each of these tools has its own specific application, but for monitoring process variables across time, the Run Chart is the most appropriate instrument.