The correct answer is $\frac{{\sum {{{\left( {x – \overline x } \right)}^2}} }}{N}$. This is also known as the variance of a set of data. It is a measure of how spread out the data are. A low variance indicates that the data points are clustered close to the mean, while a high variance indicates that the data points are spread out over a large range of values.
Option B is the third moment about the mean, which is a measure of how skewed the data are. A positive third moment indicates that the data are skewed to the right, while a negative third moment indicates that the data are skewed to the left.
Option C is the second moment about the origin, which is not a commonly used measure. It is a measure of how spread out the data are around the origin, rather than around the mean.
Option D is not a valid expression.