This is from https://www.publicserviceprep.com/SampleTests/canarmy/TestPage_1.aspx?TESTID=496 question 6. I know the answer (C) but don't know why or the reasoning behind it. Can someone please explain to me why it is the answer?
The answer is
It alternates between removing the first column and adding a column but removing one ball.
Using that principle, you will see that the colors are not important and are just a distraction. (Which I have seen often in such puzzles)
C is the only one that satisfies that condition.