Step-by-step explanation
A function is a set of relation without repetitive domain. That means we cannot have the same value of x in a set.
Example
{(1,1),(1,2),(2,4),(3,9)}
This is not a function because the relation has same two x = 1 which does not make it a function.
{(1,1),(2,4),(3,9)}
This is a function because there are no same x-values or repetitive domain in a single set.
Back to the question, we can find which table represent a function by looking at the x-value table. If there are two same or more x-values, it is not a function.
Answer
The answer is the left bottom table as the other table have two same x-values.