ANS:B - k

Thermal diffusivity (α) is defined as the ratio of thermal conductivity (k) to the product of density (ρ) and specific heat capacity (Cp​) of a substance. It is represented by the equation: α=ρCp​k​ Given that thermal conductivity (k) is already known, we can see that thermal diffusivity is inversely proportional to ρ and Cp​. Therefore, the correct answer is: 1/k​​ So, thermal diffusivity is proportional to 1/k​.