The correct option is B λ=1√2.πσ2N∗
Mean free path, λ=Average distance travelled per unit time (uavg)Number of collision made by single molecule in unit time
λ=uavgZ1
Z1=√2πσ2N∗uavg
Formula for mean free path is λ=1√2.πσ2N∗
where, N∗= Number of particles per unit volume
σ=Collision diameter (2r)