Final answer:
A control chart is the appropriate tool to monitor the number of defects per unit, used in a manufacturing or production context.
Step-by-step explanation:
A control chart used to monitor the number of defects per unit is the c. This type of control chart is especially useful for understanding the performance and variation of a process over time when it comes to defects or nonconformities. Contrary to a Pareto chart, which organizes defects from the most to the least common to identify the most significant problems, and a histogram, which shows the distribution of a set of data, the control chart is a dynamic tool that helps to monitor process stability. Options like scatter plot can be useful to find correlations between two variables but are not designed to track defects per unit over time.
Considering the need to track the frequency of defects in a manufacturing process, as suggested by the factory manager wanting to understand how many products are defective in comparison to how many are produced (Try It Σ 11.1), the control chart is the tool of choice. This tool allows the manager to not only spot the actual number of defects but also to detect trends and take corrective actions before nonconformities become critical.