If it is a linear function, then the same difference in x produces the same difference in y.
Top left: The difference in y is always 2 even though hte difference in x is not.
Not a linear function.
Bottom left: Difference in x is always 1, but difference in y is 2 or 3. Not a linear function.
Top right: Difference in x is always 2. Difference in y is always 2. This is a linear function.
Bottom right: Difference in x is always 2, but difference in y is 2 or 3. Not a linear function.