D, should be investigated because an assignable cause of variation might be present.
Explanation:
An R-chart is a control chart that is used to observe the variability of a process when measuring or analyzing a small subgroup of the process. The R-chart has a control limit and the position of a process variability has its own effect on the process.
In a R-chart, the center line of each subgroup analyzed is usually the expected value of the range of the process.
When the process variability is below the center line, the process should be investigated as there might be a cause of variation being below the center line.
