The answer is C. To find the average or mean, you add up all the values, then divide by how many values there are. 76+99+86=261. 261/3=87. So, I looked at the options and did the math. 76+99+86+88=349. 349/4=87.25. When I used 90, I got 87.75. When I used 99, i got 90 total. So, C is your answer. Hope I helped!
