Reaction 3 is different from the rest and it is because a single product is synthesized.
Option 2
Explanation:
The reaction given in 1,2 and 4 are dissociation reaction where a single compound is getting split into two compounds. So all those three reactions are exothermic as they are breaking the bonds leading to release of energy to the surrounding. But the reaction 3 is a formation reaction, where two chemical compounds are forming a single compound.
So in this case, new bonds are formed leading to absorption of energy from the surrounding. So this is an endothermic reaction. Thus, reaction 3 is different from the rest and it is because a single product is synthesized.
