2) which of the following is a synthesis reaction?
(1) cu + 2agno3 → cu(no3)2 + 2ag
(2) 2cu +02→ 2cuo
(3) cucoo3 → cuo + co2
(4) cuo + h2 → cu + h2o

(4) CuO + H₂ → Cu + H₂O.

Explanation:

(1) Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag.

It is considered as a single replacement reaction that Cu replaces Ag in its salt AgNO₃.

(2) 2Cu + O ₂ → 2CuO.

It is a combustion reaction that Cu reacts with oxygen results CuO.

(3) CuCO₃ → CuO + CO₂.

It is a decomposition reaction that CuCO₃ is decomposed to CuO and CO₂.

(4) CuO + H₂ → Cu + H₂O.

It is considered as a synthesis reaction that two reactants CuO and hydrogen are reacted to produce Cu and H₂O.

So, the right choice is: (4) CuO + H₂ → Cu + H₂O.

if you mean hydrogen and oxygen then water is the product