Expression of thermal diffusivity of a substance in terms of therma conductivity,mass density and Specific heat?

Question

Which one of the following expresses the thermal diffusivity of a substance in terms of thermal conductivity of a substance (k), mass density (r) and specific heat (c)?                (IES-06)

(a) k² pc                       (b) 1/p kc        

(c) k/pc                        (d) pc/k²