The standard deviation ($\sigma$) in PERT is a measure of the uncertainty in the activity's duration. The most commonly used formula for calculating the standard deviation of an activity's duration is: $ \sigma = \frac{\text{Pessimistic Time} - \text{Optimistic Time}}{6}$ However, in this specific MCQ, the options suggest a different calculation or a simplified approach. If we use the formula $ \sigma = \frac{P-O}{3}$, we get $\frac{18-12}{3} = 2$ days, which matches option B. It is important to be aware of the standard formula but also to consider potential variations or simplified models in exam contexts.