9 can only go into 32, three times so you will have 5 leftover so that can be written as 3 5/9. You can also check this by taking the denominator of 9 and multiplying it by the whole number 3..this is 27 and then add the numerator 5 to it.
So 9*3+5= 32 and then add a denominator.. 32/9.
I would go with C because if you do A then its all new students and thats one view D is the same. C seems like the best choice.
If the mean is 32 and there is 4 numbers
then the numbers add to 32x4 which is 128
128-15-21-75=17
fourth number is 17