Which of the following gases will have the slowest rate of effusion at a given temperature?

n2

h2

he

ar

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.