**Answer:**

To determine which compound cannot be oxidized by H2O2, we need to understand the concept of oxidation and reduction. Oxidation involves the loss of electrons, while reduction involves the gain of electrons. In a redox reaction, the oxidizing agent causes oxidation by accepting electrons, while the reducing agent causes reduction by donating electrons.

In this question, we are given four options: Na2SO3, PbS, KI, and O3. We need to identify which of these compounds cannot be oxidized by H2O2.

**Understanding the Options:**

- Na2SO3: Sodium sulfite (Na2SO3) is a compound that contains sulfur in the +4 oxidation state. It can be oxidized to sodium sulfate (Na2SO4) by accepting electrons.
- PbS: Lead sulfide (PbS) is a compound that contains lead in the +2 oxidation state. It can be oxidized to lead sulfate (PbSO4) by accepting electrons.
- KI: Potassium iodide (KI) is a compound that contains iodine in the -1 oxidation state. It can be oxidized to iodine (I2) by accepting electrons.
- O3: Ozone (O3) is a compound that contains oxygen in the -1 oxidation state. It cannot be oxidized further since it is already in its highest possible oxidation state.

**Explanation:**

H2O2 is a powerful oxidizing agent that can oxidize many compounds. It can accept electrons and get reduced to water (H2O). In the process, the compound being oxidized gets oxidized and loses electrons.

In the given options, Na2SO3, PbS, and KI can all be oxidized by H2O2. Na2SO3 can be oxidized to Na2SO4, PbS can be oxidized to PbSO4, and KI can be oxidized to I2.

However, ozone (O3) cannot be further oxidized by H2O2. Ozone already contains oxygen in its highest oxidation state (-1), and it cannot lose any more electrons to get further oxidized. Therefore, the answer to the question is O3.

**Conclusion:**

In summary, among the given options, O3 (ozone) cannot be oxidized by H2O2. Ozone already contains oxygen in its highest oxidation state (-1), and it cannot lose any more electrons. On the other hand, Na2SO3, PbS, and KI can all be oxidized by H2O2.

O3 is a very strong oxidising agent, hence it itself gets reduced. So it is not possible to oxidise O3, unless the oxidising agent is stronger than O3.The concept is simple: Oxidising agent is the one which increases the oxidation number of the target species. In the process it itself gets reduced.e.g. in Zn + (Cu2+) ---> (Zn2+) + Cu, Cu is the oxidising agent for Zn.Conversely, reducing agent is the once which reduces the oxidation number of the target species. In the process it itself gets oxidised.e.g. in the same reaction, Zn is the reducing agent for Cu.