The correct answer is: B. On hillsides
Thermal belts are bands of different temperatures that can be found on hillsides. The temperature of the air decreases as you go higher in elevation, so the air on a hillside will be cooler at the top than at the bottom. This difference in temperature can create bands of different vegetation, with different plants growing in the cooler and warmer areas.
Option A is incorrect because valley floors are typically warmer than hillsides. This is because the air in a valley is trapped by the surrounding hills, which prevents it from circulating. As a result, the air in a valley can become stagnant and warm.
Option C is incorrect because mountain tops are typically the coldest areas. This is because the air at high altitudes is thinner, which means there is less air to trap heat. As a result, the air at high altitudes can become very cold.
Option D is incorrect because thermal belts are a real phenomenon that can be found on hillsides.