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.

the answer is symmetrical : )