This question is incomplete because of the absence of d diagram or the temperature of the pancakes provided as options to be placed on top of the first pancake. The pancake to be placed on top of the bottom pancake that will make it the warmest must be the pancake with the highest temperature.
The thermal energy from the top pancake (the chosen pancake) will be transferred to the bottom pancake through a process known as conduction. Here, the top pancake will lose some thermal energy and thus it's temperature will reduce while the thermal energy of the bottom pancake will increase thereby leading to an increase in temperature of the bottom pancake. This is so because during conduction, the molecules of hotter pancake on top will move faster (than the molecules of the lower pancake) and collide with the molecules of the lower pancake sparking an increase in molecular movement of the lower pancake thereby increasing the thermal energy and subsequently the temperature of the lower pancake. And since the collision is infact of a form of energy transfer (based on law of conservation of energy), the thermal energy of the hotter pancake is reduced as the lower pancake increases in thermal energy.