Heat Islands - areas of increased temperature due to a high amount of concrete - occur in:
O Mountains
O Cities
O Tropical islands
O Grasslands

Heat Islands occur in cities.

Explanation:

The cities, especially the large ones, have managed to totally transform the natural landscape and create something that can not be found in nature. Humans have created large areas with very large populations. This has resulted in the extensive building, pilling up of certain materials, industry, traffic, blocking of winds, pollution, etc.

All of the aforementioned things have managed to create a micro-climate when it comes to the cities, differing from the climate in the surrounding area. Something that is often a characterisitc of the micro-climate in the large cities is the so-called ''heat island''. Basically, the materials that dominate in the cities, like concrete, asphalt, and glass, all manage to amplify the effect of the sun, accumulating and radiating heat, making the cities unbearably hot in some parts of the year.