From the given box and whisker's plot, we have

Now, we know that there is half data distributed between the lower and upper quartiles.
Also, we have to find that how many schools have participated for 3 to 7 years. It means we need to find the number of years in between Q1 and Q3.
The total number of schools is 16.
Hence, number of schools have participated for 3 to 7 years is given by half of the total number of schools.
=

Therefore, 8 schools have participated for 3 to 7 years.
D is the correct option.