The first value of each pair in the set represents an input.
For a relation to be a function, it has to fulfil the restriction that says that for a function, every input value has only one associated output.
From the available options, we cannot select 2, 3, 8 or 9, because they already have an associated output, and selecting any of them will associate a second output, preventing the relation from being a function.
With that being said, the only option that makes the relation a function is 4.