In the following redox reaction, which compound is acting as a...

In the given reaction, the oxidation state of nitrogen in $N_2H_4$ is $-2$ and it changes to $0$ in $N_2$. Since the oxidation state of nitrogen increases, $N_2H_4$ undergoes oxidation.

The oxidation state of oxygen in $H_2O_2$ is $-1$ and it changes to $-2$ in $H_2O$. Since the oxidation state of oxygen decreases, $H_2O_2$ undergoes reduction.

The species that undergoes oxidation acts as a reducing agent. Therefore, $N_2H_4$ is the reducing agent.

Answer: $N_2H_4$