Final answer:
Deserts are known for their extreme heat and low precipitation, making them have the highest average temperatures compared to coastal regions, mountainous areas, and temperate forests.
Step-by-step explanation:
The environment with the highest average temperatures would typically be a desert. Deserts are characterized by having very low precipitation and high temperatures, especially during the daytime. Unlike coastal regions that benefit from the regulation of temperatures by nearby water bodies, deserts do not have this moderating influence. Also, temperatures in mountainous areas are lower due to the increase in altitude, and temperate forests have moderate temperatures with defined growing seasons. Therefore, comparing these environments, deserts are generally much hotter.