Relation 1 IS a function.
Each input is used only once, so each input is paired with exactly one output.
Relation 2 is NOT a function.
The input of 3 has two different outputs.
Relation 3 IS a function.
Each x-value is used only once, so each input is paired with exactly one output.
Relation 4 IS a function.
Each x-value is used only once, so each input is paired with exactly one output.
It's all about looking at the x-values/inputs. If there's ever a repeated x-value/input, it's not a function. If they x-values/inputs are all different, then it is a function.