The first one is not a function due to the rule that there can not be more than one x, such as there is a repeated number 1 , 2 , 1 , 4 There should not be two
x = 1 terms.
Same the same rule applies to the second option as well as the last.
Your answer is C. or the third option
x= 6, 5, 4, 1
y=6, 4, 6, 2
Hope this Helps