Final answer:
The mode of the given data set is none.
Step-by-step explanation:
The mode is the value or values that occur most frequently in a data set. In the given data set, the frequency of $20 is 1, the frequency of $15 is 1, the frequency of $10 is 1, and the frequency of $5 is 1. Since all the frequencies are the same, there is no mode in this data set. Therefore, the answer is none of the options (a) $20, (b) $15, (c) $10, or (d) $5.