Determining the Reducing/Oxidizing Agent

[305920501]

How do we know when to only consider one element as being reduced/oxidized versus an entire compound? For example, in 6K.3 part b, when we are balancing the redox reaction, we are supposed to consider H2SO3 as the reducing agent, but it is oxidized to HSO4-, which is a decrease in the oxidation state of the entire compound. Why wouldn't just sulfur be the oxidized? How do we know when one molecule like sulfur is oxidized / is the reducing agent versus when it is the entire compound?

[Kenneth Vu 3B]

When we find the agent of something it's normally the whole compound.