Question

Which of the following combinations of reactants will result in a synthesis reaction?

Answer 1

Correct Answer: Option D i.e. SO2 + H2O

Reason: 
A synthesis reaction occurs when two or more reactants combine to form a single product. This reaction can be represented as A + B → AB.
In the present case, SO2 + H2O → H2SO3

Hence, it is an example of synthesis reaction. 

Answer 2

The first reaction from the scale Cl2 + BaF2 is decomposition, remember! that H20 should be broken down.
The third one is  metal ion substitution
The last one is decomposition again.
The second one is H2 + Cl2  has been combined in order to synthesize  2HCl, so that's the answer.