Answer: (g)
Step-by-step explanation:
A reacting substance can either be a solid, liquid or gas.
For instance:
A(s) + B(g) ---> AB(l)
In the reaction above, the reactants are "A", a solid with state symbol (s); while "B", a gas with state symbol (g); both form a product "AB", a liquid with state symbol (l).
Thus, (g) is the state symbol that shows the reacting substance was a gas.