Answer: d. positive square root of the variance.
Explanation:
Standard deviation is used to show the risk on a set of stocks and it is calculated by taking the square root of the variance of those same returns.
The process of finding the standard deviation is to therefore find the average return first and then find the variance of these returns. At this point, the variance will be positive as it is a squared value. Once this value is then taken to its square root, you will have your standard deviation.