Question

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

Answer 1

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
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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.