An equation represents a represents only when there is a unique value of x for every value of y. when it represents u as a function of x.
i.e if we draw lines parallel to y axis and it cuts the curve two or more than two times at a particular value of x then it doesn't represent a function at all.
And as we can see in the given tables,
1.) 12.23 is the value of x for both 5.4 and 6.4
2.) 0.31 is the value of x for both 55 and 59
4.) 20 is the value of x for both 245 and 255
So, the only table representing y as a function of x is table 3.