Mean Deviation:
Mean deviation is a measure of dispersion that calculates the average distance between each data point and the mean or average of the data set. It gives an idea of how spread out the data points are from the mean. The mean deviation is calculated by taking the absolute value of the differences between each data point and the mean, summing them up, and dividing by the total number of data points.

Explanation:
The arithmetic mean of the numerical values of the deviations of items from some average value is called the mean deviation. Let's understand why mean deviation is the correct answer for this question.

Standard Deviation:
Standard deviation is a measure of dispersion that calculates the average amount of variation or spread in a set of data. It is the square root of the variance, which is the average of the squared differences between each data point and the mean. While standard deviation is a commonly used measure of dispersion, it is not the correct answer for this question.

Range:
Range is a measure of dispersion that calculates the difference between the maximum and minimum values in a data set. It gives an idea of how spread out the data points are, but it does not consider the deviations from the mean. Therefore, range is not the correct answer for this question.

Quartile Deviation:
Quartile deviation is a measure of dispersion that calculates the difference between the upper quartile and the lower quartile in a data set. It provides information about the spread of the middle 50% of the data, but it does not consider the deviations from the mean. Hence, quartile deviation is not the correct answer for this question.

Mean Deviation:
Mean deviation, also known as average deviation, calculates the average distance between each data point and the mean. It takes into account the deviations of each data point from the mean. By finding the absolute value of each deviation, summing them up, and dividing by the total number of data points, we can obtain the mean deviation. Therefore, mean deviation is the correct answer for this question.

Conclusion:
The arithmetic mean of the numerical values of the deviations of items from some average value is called the mean deviation. It is a measure of dispersion that considers the deviations of each data point from the mean. It is calculated by finding the absolute value of each deviation, summing them up, and dividing by the total number of data points.