There are three measures of central tendency. Mean, median, and mode.

The mean is a typical average. You add up all the numbers then divide by how many numbers you have. In this problem:

83 + 88+87+83+90=431 then

431 divided by 5 = 86.2

The median is the middle number when all the numbers are put in order. In this problem:

83,83,87,88,90

There are five numbers so the third one in from either side is the median, in this case 87.

The mode is the number that appears most often in the group of numbers. In this problem:

All of the numbers appear once, except 83, which appears twice, therefore the mode is 83.

In this problem Nicholas would be best off with the median because it gives him the highest score (87).