Final answer:
The ammonium ion (NH4+) forms a neutral compound with a group 1A monatomic ion in a 1:1 ratio, as it has a 1+ charge.
Step-by-step explanation:
The question asks which polyatomic ion forms a neutral compound when combined with a group 1A monatomic ion in a 1:1 ratio. The answer is the ammonium ion (NH4+), as this ion has a 1+ charge which will balance out the 1- charge of a Group 1A ion like sodium (Na+) or potassium (K+) when combined in a 1:1 ratio. The other options, carbonate (CO32-), nitrate (NO3-), and phosphate (PO43-), do not form neutral compounds with a 1:1 ratio as their charges are either 2- or 3-, which requires more than one group 1A ion to neutralize the charge.