Which environment is most conducive for coal formation?

Dense forests in wetland areas are indeed the most conducive environment for coal formation. This is primarily because coal originates from the accumulation of plant material, specifically the remains of large amounts of vegetation that can be found in such environments.

In wetland areas, a rich supply of plant matter is produced, particularly in dense forests, where decaying organic material can become buried under sediment. The anaerobic conditions typical of wetlands help prevent the complete decomposition of this organic matter, allowing it to be compacted and transformed over millions of years into peat, and eventually into coal through further geological processes.

The conditions in dense forest wetland areas provide the necessary combination of abundant plant material and suitable anaerobic conditions for coal formation, unlike the other environments listed. High mountain regions with volcanic activity are not conducive due to the lack of suitable plant material and the activity that would disrupt any formation processes. Desert areas lack sufficient vegetation and moisture, essential for the growth of plants that would contribute to coal deposits. Open plains with scattered trees may have some plant material, but they would not have the dense accumulation and anaerobic conditions necessary to form coal effectively.