The correct answer is C. 10 days.
The standard deviation is a measure of how spread out the data is. A low standard deviation indicates that the data points tend to be very close to the mean, while a high standard deviation indicates that the data points are spread out over a large range of values.
In this case, the standard deviation is 2 days. This means that 68% of the time, the project will be completed between 8 and 12 days. 95% of the time, the project will be completed between 6 and 14 days. And 99.9% of the time, the project will be completed between 4 and 16 days.
However, this does not mean that the project will always be completed within these ranges. There is always a chance that it will take longer or shorter than expected. For example, if the project is very complex or if there are unexpected delays, it could take longer than 16 days to complete. On the other hand, if the project is very simple or if everything goes according to plan, it could be completed in less than 4 days.
The expected time of completion of the project with 99.9% probability is 10 days. This means that if the project were to be repeated many times, we would expect it to be completed in 10 days or less 99.9% of the time.