TO BE A FUNCTION: No vertical (up and down) line can cross more than one point of your graphed line. This means that your X coordinates (the first number in each pair) cannot show up more than once in your set of ordered pairs.
Option C has the unique X coordinates of 2, 9, and 1. When graphed, your ordered pairs create an upside down V. Because there is no vertical line you could possibly draw that would intersect your graph in more than one place, you've got yourself a function.
Check out this website for some quick practice with visual aids: http://math.uww.edu/~mcfarlat/141/fn_test.htm