The correct statement comparing synthesis and decomposition reactions is:
Synthesis and decomposition reactions are the opposites of each other. Synthesis reactions are in the form A + B → AB while decomposition reactions are in the form AB → A + B.
In synthesis reactions, two or more simple substances combine to form a more complex substance (A + B → AB). In decomposition reactions, a complex substance breaks down into simpler substances (AB → A + B). Therefore, they are indeed opposites.