Final answer:
In the given redox reaction, SO2 is the reducing agent as it is oxidized, having its oxidation state increased.
Step-by-step explanation:
To find the reducing agent in a redox reaction, one must identify which substance is being oxidized because the species that is oxidized is the reducing agent. In the reaction VO43-(aq) + SO2(g) + 2H+(aq) → VO2+(aq) + SO42-(aq) + H2O(l), we need to determine the changes in oxidation states of the elements involved to identify the oxidizing and reducing agents. When balancing a redox reaction, it is essential to remember that oxidation is the loss of electrons, resulting in an increase in oxidation state, while reduction is the gain of electrons, leading to a decrease in oxidation state. For the SO2 to convert to SO42-, sulfur is oxidized, meaning its oxidation state is increasing. Hence, SO2 serves as the reducing agent here because it is giving up electrons.