Final answer:
The gas with the greatest average molecular speed at 25°C would be the one with the lightest molecules, which is hydrogen gas (H2).
Step-by-step explanation:
The average molecular speed of a gas is directly related to the temperature of the gas. According to the kinetic theory of gases, at a given temperature, lighter molecules move faster than heavier ones. Therefore, the gas with the greatest average molecular speed at 25°C would be the one with the lightest molecules.
In the given options, hydrogen gas (H2) has the lightest molecules, and therefore it would have the greatest average molecular speed at 25°C.