We are looking at the x values. In order to be a function, these cannot repeat. Relation one is a function. Relation 2 is also a function. Relation 3 is not a function because the domain values each occur more than once. Relation 4 is not a function because d has two outputs.
Also, sorry about before. I cannot seem to modify these posts after they are submitted. I am also not able to post a correction, so I just wanted to apologize.