To find the coefficient of mean deviation about mean, we need to first find the mean of the given data.

Finding the mean:
The first 9 natural numbers are: 1, 2, 3, 4, 5, 6, 7, 8, 9.
The sum of these numbers is: 1+2+3+4+5+6+7+8+9 = 45
The mean of these numbers is: 45/9 = 5

Finding the mean deviation about mean:
Mean deviation about mean is the average deviation of each data point from the mean.
To find the deviation of each data point from the mean, we subtract the mean from each data point.

Deviation of 1 from mean: 1-5 = -4
Deviation of 2 from mean: 2-5 = -3
Deviation of 3 from mean: 3-5 = -2
Deviation of 4 from mean: 4-5 = -1
Deviation of 5 from mean: 5-5 = 0
Deviation of 6 from mean: 6-5 = 1
Deviation of 7 from mean: 7-5 = 2
Deviation of 8 from mean: 8-5 = 3
Deviation of 9 from mean: 9-5 = 4

To find the mean deviation about mean, we take the absolute value of each deviation, add them up and divide by the number of data points.

Mean deviation about mean = (|-4| + |-3| + |-2| + |-1| + |0| + |1| + |2| + |3| + |4|)/9
= (4+3+2+1+0+1+2+3+4)/9
= 20/9

Finding the coefficient of mean deviation about mean:
The coefficient of mean deviation about mean is the mean deviation about mean divided by the mean.

Coefficient of mean deviation about mean = (20/9)/5
= 20/45
= 4/9

Multiplying this by 100 gives us the answer in percentage form.

Coefficient of mean deviation about mean = (4/9) x 100
= 44.44%

Therefore, the correct answer is option A) 400/9.