Heat transfer by convection from the bottom of the water to the top of the water.
Step-by-step explanation:
Convection takes place in case of liquids. In case of liquids the molecules of water are very loosely packed. Convection takes place in a circular form. When the liquid is heated up the water vapor goes up the cold water come to take the empty places , this way convection takes place. In this question , when the water is heated first the bottom of the water gets heated up and gradually the water at the top becomes hot.