Which of the following is a synthesis reaction?

BaO + CO2 → BaCO3

CH4 + O2 → CO2 + H2O

FeCO3 → FeO + CO2

2NH3 → 2N2 + 3H2

The reaction which is a synthesis reaction is

BaO + CO₂→ BaCO₃

Explanation

Synthesis reaction a reaction which involve combination of two or more simple substance to from a more complex substance.

BaO and Co₂ combine to form BaCO₃ (a more complex substance) therefore the reaction above a synthesis reaction.

CH₄ + O₂→ Co₂ + H₂O is a combustion reaction since it involve a hydrocarbon reacting with O₂

Both FeCo₃ → feO + CO₂ and 2NH₃ → 2N₂ + 3H₂ are decomposition reactions.