The mode in a set of numbers is the number that "occurs" the most
Lets make up a set
1,1,1,2,3,3,3,3,4,4
In this set, there are 3 number 1's, 1 number 2, 4 number 3's and 2 number 4's in this set.
There are more 3's than any other number so therefore the mode is 3
Now back to the actual problem.
There is 1 number 13, 1 number 14,1 number 16, 3 number 17's, 1 number 18, and 1 number 20 in this set.
There are more 17's in this set so the mode is 17 which is C)17