Which of the following gases has the highest average speed at 400k A)He
B)Kr
C)Ne
D)Ar

He

Explanation:

Helium will have the highest speed at the given temperature value because it has the lowest molecular mass.

According to Graham's law of diffusion "the rate of effusion or diffusion of a gas is inversely proportional to the square root of its molecular mass or density at constant temperature and pressure".

 In short, a gas with low molecular mass can diffuse or effuse more rapidly than one with a high molecular mass under the same condition.

Molecular mass of He  = 4.0026g/mol

                               Kr =  83.789g/mol

                               Ne = 20.1797g/mol

                               Ar = 39.948g/mol

He has the lowest molecular mass of the given choices.