If NH4+ is the cation, then what is the anion of it?

[HCverma]

If NH4+ is the cation, then what is the anion of it so that the cation and anion combine and make a molecule? as in HNO3, where the cation is H+ and the anion is HNO3- and these cation and anion combine, make a neutral molecule HNO3.

 

[BvU]

OH^- (for the case of NH³ and water)

 

[Borek]

In general any anion will do.

 

[PeterDonis]

If NH4+ is the cation, then what is the anion of it so that the cation and anion combine and make a molecule?

It depends on the molecule. You need to tell us which molecule you are interested in, just like you did in the example you gave:

as in HNO3, where the cation is H+ and the anion is HNO3- and these cation and anion combine, make a neutral molecule HNO3.

Here we know the cation and anion because you told us the molecule. We can't know the anion that goes with NH4+ unless you tell us what molecule you are interested in.