Which statement about diffusion paths in a polycrystalline metal is correct?

Answer: B. The lower the temperature, the larger the contribution of grain boundary diffusion relative to lattice diffusion

The more disordered the atomic packing along a path, the more easily atoms jump, so the activation energy generally increases in the order surface diffusion < grain boundary diffusion < lattice (volume) diffusion. The choice giving lattice < grain boundary < surface has this order reversed. Paths with lower activation energy lose less of their diffusion coefficient as the temperature falls, so the relative contribution of grain boundary diffusion grows at lower temperatures. Finer grains mean a larger total grain boundary area, so the contribution of grain boundary diffusion actually increases. This is why grain boundary diffusion becomes a problem in low-temperature, fine-grained systems such as solder joints and thin-film interconnects.