Which of the Following Is the Definition of "Standard Deviation

Which of the following is the definition of "standard deviation"?

A) an average that is calculated by adding up a set of quantities and dividing the sum by the total number of quantities in the set
B) a statistical test that assesses how likely it is that a study's results occurred merely by chance so that the researcher knows that the probability that the difference is "real" is overwhelming
C) a commonly used measure of variability that indicates the average difference between scores in a distribution and their mean
D) a statistical procedure that allows researchers to draw implications about how statistically meaningful a study's results are