In a function, every input value can be related to only one output.
In each table, the inputs are in the first column and the outputs in the second column.
In consequence, the first two tables show functions.
In the case of the third table, the input of 2 is related to two different outputs, then this table is not a function.
In the case of the fourth table, the input of 1 is related to two different outputs, then this table is not a function.