Answer: Option (d) is the correct answer.
Step-by-step explanation:
The given chemical reaction is as follows.

When we use the subscript (s), the it means that the substance is solid. When we use the subscript (aq), then it means that the substance is aqueous. When we use the subscript (g), then it means that the subscript is gas.
Thus, in the given chemical reaction
is the gas.