Standard deviation is equal to:

A$)$ the probability$-$weighted average of the product of each variable's difference from its mean.

B$)$ the square root of the average value of squared deviations from the mean.

C$)$ the probability$-$weighted average of possible outcomes.

D$)$ the average value of squared deviations from the mean.

E$)$ the square root of the probability$-$weighted average of possible outcomes.