What is the reasoning behind setting the upper and lower control limits $3$ from the$$ mean?

Part $2$

A$.$

We know that$99\mathrm{.}73\%$ of natural variation is contained within $3,$ and thus any data point outside of the limits is likely due to assignable variation

B$.$

The limits of the Z table are set at $3$

C$.$

$95\mathrm{.}45\%$ of the$$ time, the sample averages will fall within $3$ if the process has only natural variation

D$.$

We know that$99\mathrm{.}73\%$ of assignable variation is contained within $3,$ and thus any data point outside of the limits is likely due to natural variation