For each reaction, give the oxidation numbers before and after for the elements that change, name…

Step 1. (a). Zinc goes $0 \rightarrow +2$, so it is oxidised and Zn is the reducing agent. Hydrogen goes $+1 \rightarrow 0$, so it is reduced and HCl is the oxidising agent.

Step 2. (b). Sulfur in $\text{H}_2\text{S}$ goes $-2 \rightarrow 0$ (oxidised), so $\text{H}_2\text{S}$ is the reducing agent. Sulfur in $\text{SO}_2$ goes $+4 \rightarrow 0$ (reduced), so $\text{SO}_2$ is the oxidising agent.

Step 3. (c). Nitrogen in $\text{NO}_2$ is $+4$. It goes to $+3$ (reduced) and to $+5$ (oxidised). $\text{NO}_2$ is both agents.

Step 4. (d). Iron goes $+3 \rightarrow 0$ (reduced), so $\text{Fe}_2\text{O}_3$ is the oxidising agent. Carbon goes $+2 \rightarrow +4$ (oxidised), so CO is the reducing agent.