Standard deviations are used to measure the variability of the set
The reason they choose the standard deviation as the measure of variability is (a) when plotted, the data set had a normal distribution
When a data is distributed, the data can be measured using the median, the mean or the standard deviation.
If the data is a normal distribution, then the best measure of variability to use is the standard deviation
Hence, the true statement is (a) when plotted, the data set had a normal distribution
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