The correct answer is: Hydrogen gas.
Hydrogen gas is produced by the reaction of acids upon metals. This is because acids are electron-rich molecules, and metals are electron-poor atoms. When an acid reacts with a metal, the hydrogen atoms in the acid are attracted to the metal atoms, and they are transferred to the metal. This results in the formation of a salt and hydrogen gas.
For example, when hydrochloric acid reacts with zinc, the following reaction occurs:
$$HCl(aq) + Zn(s) \to ZnCl_2(aq) + H_2(g)$$
In this reaction, the hydrogen atoms from the hydrochloric acid are transferred to the zinc atom, forming zinc chloride and hydrogen gas.
The other options are incorrect because they are not produced by the reaction of acids upon metals. Carbon dioxide is produced by the reaction of acids with carbonates or bicarbonates. Nitrogen is produced by the reaction of acids with ammonia. Oxygen is produced by the electrolysis of water.