ANS:D - Air

The overall heat transfer coefficient (U) is influenced by several factors including the properties of the fluid and the configuration of the heat transfer surface. Generally, fluids with lower thermal conductivity and/or lower convective heat transfer coefficients will result in lower overall heat transfer coefficients. Among the options provided:

• Dowtherm is a heat transfer fluid commonly used in industrial applications. It typically has moderate thermal conductivity and convective heat transfer coefficients.
• Molten sodium has relatively high thermal conductivity but its convective heat transfer coefficient may vary depending on the flow conditions.
• Water typically has high convective heat transfer coefficients due to its high thermal conductivity and density, particularly compared to gases.
• Air generally has lower thermal conductivity and convective heat transfer coefficients compared to liquids like water.

Based on this, air is likely to have the lowest overall heat transfer coefficient among the options provided, especially when compared to Dowtherm, molten sodium, and water which are liquids with generally higher thermal conductivities and/or convective heat transfer coefficients.