Final answer:
A c-chart is used for monitoring the number of defects per unit, which is useful in quality control processes when the number of units or opportunities for defects is constant.
Step-by-step explanation:
The c-chart is used for B) number of defects per unit. It is a type of control chart that is utilized to monitor the count of nonconformities or defects in an item or process over a set period of time. The c-chart is suitable when the number of opportunities or units is constant and the probability of a defect occurring is the same for each unit. For instance, if we are considering an assembly line producing a single type of product, we could use a c-chart to track the number of defects in each product.
When analyzing the number of defects in a particular context, as in the given example regarding the survey results from Table 11.6 or the expected number of defects in Table 11.5, the c-chart helps to understand the variation in a process and identify whether that process is under control. It does not track percent defective, mean values, or ranges, which are monitored using other types of control charts such as p-charts, α-charts, and R-charts, respectively.