Question

which part of the room is coolest according to the thermal image?
the windows and their frames
the walls around the windows
the radiators under the windows

Explanation:

In thermal imaging, cooler areas are represented by cooler colors (like blue). Looking at the thermal image, the windows and their frames are shown in the coolest - colored part (blue) compared to the walls around the windows (which would be in a warmer color range than the windows if we consider the thermal scale) and the radiators under the windows (radiators are for heating, so they should be in warmer colors).

Answer:

the windows and their frames