Final answer:
When the oxidation number of an atom in a compound increases, it means the atom has lost electrons and has undergone oxidation. As a result, the compound in which the oxidation number has increased acts as a reducing agent, delivering electrons to another substance.
Step-by-step explanation:
If an oxidation number of an atom in a compound increases from +2 to +4, this means the atom has lost electrons, which is characteristic of oxidation. During this process, the compound in which the atom resides is acting as a reducing agent, because it is providing electrons to another species. Hence, when an element in a compound has an increase in oxidation number, it does not gain electrons; instead, it loses them. Moreover, the process does not involve the gain of protons.
In the context of a redox reaction, an increase in the oxidation number indicates oxidation, whereas a decrease signifies reduction. For example, in the oxidation of carbon to carbon dioxide (C + O2 → CO2), carbon is oxidized as its oxidation number increases from 0 to +4. Here, carbon dioxide (CO2) is the product of carbon oxidation, and in this context, the carbon would be the reducing agent because it provides electrons to the oxygen.
Therefore, the correct answer to the question is that the compound is a reducing agent because its oxidation state is increasing, which correlates with the loss of electrons.