Final answer:
The mode of the data set is the number that appears most frequently, which in this case is 4, since it appears nine times.
Step-by-step explanation:
To find the mode of a data set, we look for the number that appears most frequently.
In the data provided from a poll of 30 students about the number of classes they are taking this term, we have the following counts:
- 1 appears four times.
- 2 appears five times.
- 3 appears eight times.
- 4 appears nine times.
- 5 appears four times.
The number that appears the most frequently is 4, as it appears nine times. Therefore, the mode of this data set is 4.