Here in this example it says which compound has oxygen with the lowest oxidation state. If we take a look at the first one, we have Na, O₂. We have one group 1A element with two oxygens. Remember that pattern tells us we have a superoxide. And if you have a superoxide, that means the oxidation number of oxygen is -12.

For the next one we have CO₂. Now it's not a superoxide. For it to be a superoxide, we would need one group 1A element with the two oxygens. Carbon is not in Group 1A, it's in Group 4A, so it's not a superoxide. Next, it's not a peroxide either. To be a peroxide, you need two group 1A elements with two oxygens. So it's not a peroxide, it's not a superoxide. Therefore oxidation number is -2.

For the next one we have C₂O₂. That's two Cs which are in Group 1A with two oxygens. That fits the definition of a peroxide. This is a cesium peroxide. So with oxidation number for oxygen would be -1.

And then finally we have O₂. This is the standard or natural state of oxygen. Remember, an element found in its standard or natural state has an oxidation number of 0. So out of all our choices, the one that has the lowest oxidation number, the one with the most negative value would be CO₂.

So just remember, if we have specific rules for that given element, utilize them to find the correct answer.