Under standard conditions, which element will be most easily vaporized? Now when we say most easily vaporized, that means which one becomes a gas most easily and between boiling point and melting point. This means we're talking about boiling point because that's where we go from a liquid to a gas. The one that's most easily changed into a gas would be the one with the lowest boiling point.
Now if we take a look at our options here, we have carbon, silicone, potassium rubidium and cesium. If you look at your periodic table, remember that carbon and silicone both can do covalent network bonding. This results in higher than normal boiling points and melting points, so those would not be our answers.
If we look at potassium orbidium and Cessium looking on the periodic table, you'll see that they're all in Group 1A. So we have potassium, rubidium, and cesium. And remember, for groups 1A to 4A, as we go up into the right of the periodic table, our boiling point increases. Here we're looking for the lowest boiling point, so that means the one lowest down, which would be cesium. So here our final answer would be option E.