The reagents for BaCO₃ is
BaO and CO₂
Explanation
Reagent is a substance that bring about a chemical reaction when added to a system.
Some reagent may be added to see if a reaction has occurred.
BaO and Co₂ are reagent since they react to produce BaCO₃ as below
BaO(s) + CO₂(g) → BaCO3(s)
