ANS:A - Molten sodium (a coolant used in fast breeder reactor)

The Prandtl number (Pr) is defined as the ratio of momentum diffusivity (kinematic viscosity) to thermal diffusivity. A lower Prandtl number indicates that thermal diffusivity is relatively higher compared to momentum diffusivity, meaning that thermal boundary layers are thinner compared to momentum boundary layers. Let's compare the Prandtl numbers of the given substances:

1. Molten sodium (a coolant used in fast breeder reactor):
   • Molten sodium typically has a low viscosity and high thermal conductivity. It also has relatively low specific heat capacity. Therefore, its Prandtl number tends to be low.
2. Water:
   • Water has a relatively high viscosity and thermal conductivity compared to molten sodium. It also has a moderate specific heat capacity. As a result, water tends to have a higher Prandtl number compared to molten sodium.
3. Transformer oil:
   • Transformer oil typically has higher viscosity and lower thermal conductivity compared to water and molten sodium. It also has a moderate specific heat capacity. Therefore, transformer oil tends to have a higher Prandtl number compared to water and molten sodium.
4. Dilute H2SO4:
   • Dilute sulfuric acid (H2SO4) is typically a liquid with properties similar to water. Its Prandtl number is likely to be similar to that of water, but it could be slightly different depending on its specific temperature and concentration.