The polyatomic ion that forms a neutral compound when combined with a Group 1A monatomic ion in a 1:1 ratio is the hydroxide ion (OH-).

Here's why:

* Group 1A elements (alkali metals) have a +1 charge when they form ions.

* The hydroxide ion (OH-) has a -1 charge.

When these two ions combine in a 1:1 ratio, the charges cancel out, resulting in a neutral compound. For example:

* Sodium hydroxide (NaOH): Na+ + OH- → NaOH

* Potassium hydroxide (KOH): K+ + OH- → KOH

Let me know if you'd like more examples or want to explore other polyatomic ions!