To find the range of a data set, we subtract the smallest value from the largest value.

The given data set is:
20, 7, 4, 6, 21, 10, 4, 12.

1. Identify the smallest value: 4
2. Identify the largest value: 21

Now, calculate the range:
$\text{Range} = 21 - 4 = 17$

The correct answer is 17.

Would you like any further details or explanations?

Here are five related questions for practice:

1. What is the median of this data set?
2. How would you calculate the mean of the given data?
3. If an additional value of 15 is added to the data set, what would the new range be?
4. What are the lower and upper quartiles of this data set?
5. How does the range change if the maximum value is decreased by 3?

Tip: The range provides a measure of data variability, but it can be sensitive to outliers. Always consider using other measures like the interquartile range for a more robust analysis.