The correct answer is C. 11.58 . The CSSBB source gives the control-limit formula for an X̄-R chart as:

UCL for X̄ chart = X̄̄ + A2R̄

The same CSSBB control-chart factor table shows that for subgroup size n = 5 , the constant A2 = 0.577 (often rounded to 0.58 in summary tables). Using the values provided:

UCL = 10.66 + (0.577 × 1.59)

UCL = 10.66 + 0.91743

UCL = 11.57743

Rounded to two decimal places, the upper control limit is 11.58 . This matches option C .

This is a standard Control phase calculation in CSSBB because control charts are used to determine whether a process is stable over time. The X̄ chart monitors changes in the process mean, while the R chart monitors within-subgroup variation. Since the problem specifically asks for the X̄-chart upper control limit, only the X̄-R chart formula and the correct A2 factor are needed. Therefore, the verified answer is 11.58 .