Mode, is talking about which number shows up the most times in a set of number.
For example, the number 1 will be the mode in the set [1,1,1,2,2,3].
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Now.. looking at your question, first order the numbers.
[0, 1, 1, 2, 2, 2, 2, 4, 4, 5, 6, 6, 6, 6, 7, 8, 8, 9, 10, 12]
After re-ordering, we can see that both 2 and 6 shows up 4 times, two more than the next numbers that have the next largest amount of numbers (1, 4 and 8, each having 2 numbers)
However, now you are adding another 6 into the set.
[0, 1, 1, 2, 2, 2, 2, 4, 4, 5, 6, 6, 6, 6, 6, 7, 8, 8, 9, 10, 12]
Now, 6 shows up 1 more time than the next largest set of number, 2.
6 will be your new mode.
C. The mode will change to one number is your answer, and that number is 6
hope this helps