ANS:D - sum

The overall resistance for heat transfer through a series of flat resistances is the sum of the resistances. Explanation:

• When heat transfer occurs through multiple resistances in series, the total resistance to heat flow is the sum of the individual resistances.
• This concept follows from the analogy to electrical circuits, where resistors in series add up to the total resistance.
• In heat transfer analysis, each resistance represents a different mode of heat transfer (e.g., conduction, convection, radiation) or a combination of them, and the total resistance is calculated by summing up these individual resistances.
• Therefore, to determine the overall resistance for heat transfer through a series of flat resistances, you simply add together the resistances of each component.