The first one (up and lef) does represent a linear function because y increments a constant amount (5) as x increments a constant amount (1).
The second one (up and right) does represent a linear function because y decreases a constant amount (3) when x increases a constant amount (1).
Th third one (bottom and left) does represent a linear function because y increases a constant amount (1) when x increases a constant amount (1).
The fourth (bottom and right) does not represent a linear function because the change is y is not constant when x increases a constant amount.