ANSWER
First table.
EXPLANATION
The table that represents a linear function has a constant difference in both the x-values and the y-values.
As for the x-values, there is a constant difference of 1 for all the tables.
From, the first table, we can see that, the difference in y-values is:

For the second table,

The third table too,

The fourth table too,

Hence the choice is the first table.