Answer: Argon (Ar).
Effusion is the escape of a gas through an opening considerably smaller than the mean free path of the molecules.
There is a formula that states that the rate of effusion is inversely proportional to the square root of the Molar Mass, at fixed temperature.
Then, the gas with the highest molar mass will have the lowest effusion rate.
Here are the molars masses of the four gases:
N2 = 2*14 g/mol= 28 g/mol
H2: 2 * 1g/mol = 2 g/mol
He: 4 g/mol
Ar: 40 g/mol
Then, Ar will exhibit the slowest rate of effusion at the same temperature.
