What is the median of the data set 4, 7, 7, 9, 12, 15, 7, 20?

First order the data: $4, 7, 7, 7, 9, 12, 15, 20$. There are $8$ values, so the median is the mean of the 4th and 5th values: $\frac{7+9}{2}=8$, choice B. Choice A ($7$) is the mode, the most frequent value. Choice C ($9$) takes only the 5th value rather than averaging the two middle values. Choice D ($10.375$) is not the median; the mean of these values is $\frac{81}{8}\approx10.1$. The median must be found after sorting; using the original order gives a different, wrong middle.