The answer would be A.
Combination reaction, otherwise known as synthesis reaction, occurs when two or more substances react and form a single, more complex substance.
A combination reaction looks like this in an equation:
A + B → AB
reactants product
So from two reactants they become one product.
Let's take a look at choice A.
2H₂O(l) → 2H₂(g) + O₂(g)
reactant product(s)
Notice that from one substance, it broke down into two substances. So this is not a combination reaction. This is what you call a decomposition reaction.
