Notice that 85 = 100 - 15, and 115 = 100 + 15. The students with scores between 85 and 115 are the ones that fall within one standard deviation of the mean. The plot tells you that a total of 68% of all 1500 students fall in this range. So the answer would be 68% of 1500, or 1020 students, making the answer C.
