ANS:D - first increase and then decrease

For a given ambient air temperature, with an increase in the thickness of insulation of a hot cylindrical pipe, the rate of heat loss from the surface would decrease. Insulation acts as a barrier to heat transfer, reducing the amount of heat that can escape from the surface of the pipe into the surrounding environment. As the thickness of insulation increases, the thermal resistance to heat flow also increases, resulting in a decrease in the rate of heat loss from the surface. Therefore, the correct option is: decrease