The formula for the compound formed by NH₄⁺ (ammonium ion) and CO₃²⁻ (carbonate ion) is (NH₄)₂CO₃. Here's why:

* Balancing charges: Ammonium has a +1 charge (NH₄⁺) and carbonate has a -2 charge (CO₃²⁻). To create a neutral compound, you need two ammonium ions to balance the charge of one carbonate ion.

* Parentheses: The parentheses around NH₄ indicate that there are two ammonium ions in the formula.

Therefore, the correct formula for ammonium carbonate is (NH₄)₂CO₃.