To understand box-and-whisker plots, you have to understand medians and quartiles of data set. The concept of median is simple, it divides the data set into two parts, the upper set and the lower set, so this is something like the middle point of the data set. On the other hand, the lower quartile is the median of the lower half, and the upper quartile is the median of the upper half.
So the answer is c) Why?
Let me explain, the lower quartile in this case is 70 and the upper quartile is 80, so in the graph the median is 75 represented by the vertical line in the box. By contrast in A) the median is 65 and regarding C) and D) the median is 70.
