Statistics

Since, and

Standard deviation

We are tasked with finding the formula for the mean deviation (M.D.) of a frequency distribution about the mean.
Step 1: Understand the formula for M.D.
The formula for the mean deviation about the mean is: where:
is the frequency of the -th observation,
is the deviation of the -th observation from the mean,
is the absolute value of the deviation.

Step 2: Conclusion
Thus, the correct formula for the mean deviation about the mean is , corresponding to option (c).