Which gas is most easily solidified?

The correct choice is Carbon Dioxide, primarily because of its sublimation characteristics and phase transition properties. At standard atmospheric pressure, Carbon Dioxide transitions directly from a gas to a solid state at temperatures below -78.5 degrees Celsius, without passing through a liquid phase. This process is known as sublimation, which is unique compared to many other gases.

In contrast, while gases like Oxygen and Nitrogen can also be solidified, they require much lower temperatures and specific pressures to achieve liquefaction before solidification, making them less readily solidifiable under standard conditions. Hydrogen, while it can be solidified too, also requires very low temperatures and high pressures, making it less practical than Carbon Dioxide in typical scenarios.

Thus, Carbon Dioxide stands out as the most readily solidifiable gas under normal conditions, effectively demonstrating why this option is the best answer.