What type of chemical reaction is so3(g) + h2o(l) → h2so4(aq)?

Note: Above question is incomplete. Complete question is
What type of chemical reaction is so3(g) + h2o(l) → h2so4(aq)
a) Neutralization
b) Decomposition
c) Combination
d) Single displacement reaction

Solution:
In current context, following things may be noted
1) Neutralization reactions: In neutralization reactions, acid and base combine to form salt and water.
b) Decomposition reaction: Reaction in which a single substance broken down two or more smaller substances is refereed as decomposition reaction. Decomposition reactions are characterized be present of single molecule on reactant side. 
c) Combination reaction: In a recombination reaction two or more molecules combine to form a single product.
d) Single displacement reaction: Reaction in which an element within a compound is displaced by a another element is referred as single displacement reaction. 

Above reaction satisfies the criteria of combination reaction, hence correct answer is option C, combination. 

The equation in this question is a type of combination reaction.

COMBINATION REACTION:A combination reaction is a type of reaction in which two or more elements or compounds are chemically combined to form a single compound. According to this question, sulphur VI oxide is combined with water molecule to form sulfuric acid. The chemical equation is as follows:SO3(g) + H2O(l) → H2SO4(aq)

Since a molecule combined with another molecule to form a compound, the equation depicts a combination reaction.

Learn more about combination reaction at: link

The answer for this question is d. none of these

It will fall off and tip over.