The best combination of layers that would work best for this purpose is an inner core of steel, no air gap, and an outer layer of glass. Option A is correct.
Thermal mugs have the ability to maintain the internal body temperature and heat of their container. For the heat to be maintained for an extended period of time. The internal core must be lined with steel because it radiates less heat compared to the glass. The steel also has high conductivity and lesser specific heat to keep drinks hot for a long period of time.
Therefore, we can conclude that for thermal mugs to keep drinks hot for as long as possible, the inner core must be lined with steel, no air gap, and contain an outer layer of glass.
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