ANS:A - Liquid metal

Among the options provided, liquid metal typically has the lowest Prandtl number. The Prandtl number (𝑃𝑟Pr) is defined as the ratio of momentum diffusivity to thermal diffusivity. It quantifies the relative importance of momentum transport to thermal transport in a fluid. Liquid metals generally have very high thermal conductivity (high thermal diffusivity) compared to their momentum diffusivity. This results in a very low Prandtl number for liquid metals. In contrast, aqueous solutions, water, and lube oil typically have higher Prandtl numbers compared to liquid metals due to their lower thermal conductivities and higher viscosities. Therefore, among the options provided, liquid metal is likely to have the lowest Prandtl number.