Final answer:
Oxygen gas will effuse the slowest out of hydrogen, oxygen, nitrogen, and helium due to its higher molar mass relative to hydrogen and helium, and being slightly higher than nitrogen as per Graham's law.
Step-by-step explanation:
The question pertains to which of the gases hydrogen (H2), oxygen (O2), nitrogen (N2), or helium (He) will effuse the slowest at the same temperature. According to Graham's law, the rate of effusion of a gas is inversely proportional to the square root of its molar mass. Helium, being the lightest, will effuse the fastest. Oxygen and nitrogen have similar molar masses, but oxygen's is slightly higher, so it will effuse slower than nitrogen. Hydrogen, with the lowest molar mass, will effuse very quickly. Therefore, oxygen (O2) will effuse the slowest among these gases.