What is the mode of the following data set: 2, 4, 6, 4, 7, 4, 8, 9?
Answer: B
The mode is the value that appears most frequently in a data set.
In the set {2, 4, 6, 4, 7, 4, 8, 9}, the number 4 appears three times, which is more than any other number.
Therefore, the mode is 4.