Correct option is (c) Damkohler number
To explain I would say: Damkohler number depends on the rate of the reaction and diffusion rate and is given by,
Da=(maximum rate of reaction)/(maximum rate of diffusion)=\(\frac{V_{max}}{K_L [S_b]}\), where
Vmax = Maximum rate of reaction
KL = Mass transfer coefficient (cm/s)
Sb = Substrate concentration in bulk liquid (g/cm^3)
Damkohler number has a significant effect on diffusional resistance. If the Da >> 1, the diffusion rate is limiting. Whereas if the Da ≪ 1, the reaction rate is limiting. For a = 1, the diffusion and the reaction resistances are comparable.