You will notice that the first answer decomposes into two seperate products with 1 reactant, so since this is only true of decomposition reactions, that is not the answer.
H2+Cl2 yields 2HCl could be the answer, as it starts with 2 reactants and ends with one product, something found in synthesis reactions.
The third answer is incorrect because even though there are no decompositions of single products, the metal exchanges positions, something that only occurs in decomposition reactions.
In the fourth answer is also incorrect because the methane is oxidized. This only happens in decomposition reactions.
The correct answer, then, is B.
