Which of the following must be one of the numbers in a data set?
Answer: C
The mode is, by definition, the most frequent value *in the data set*. Therefore, it must be one of the numbers present in the set.
The mean and median can be values that do not actually appear in the data set. For example, for the set {2, 4}, the mean is 3 and the median is 3, neither of which is in the set.