Which of the following acts as both a reducing and an oxidizing agent?

A substance that can act as both a reducing agent and an oxidizing agent is known as an "oxidizing-reducing agent" or a "redox agent". In a redox reaction, one substance loses electrons (is oxidized) while another substance gains electrons (is reduced). Therefore, to determine which of the given options acts as both a reducing and an oxidizing agent, we need to find a substance that can both lose and gain electrons. Out of the given options, only sulfur dioxide (SO2) can act as both a reducing and an oxidizing agent. In the presence of a strong oxidizing agent, such as potassium permanganate (KMnO4), sulfur dioxide can be oxidized to form sulfuric acid (H2SO4), while the potassium permanganate is reduced. Conversely, in the presence of a strong reducing agent, such as zinc (Zn), sulfur dioxide can be reduced to form sulfurous acid (H2SO3), while the zinc is oxidized. Therefore, the answer is "SO2".