Which environment of the ancient world was the kind where our current coal beds were formed

The correct answer is - tropical rainforest.

The coal beds that are used in the present are all result of a certain period of time in the geologic history when the Earth was much warmer, wetter, and was covered with dense rainforests.

The period in question is the Carboniferous period, lasting from 360 to 300 million years ago. The trees are one of the biggest natural storage of carbon, so when the tropical rainforests of the Carboniferous collapsed, because of the environment, big portion of the trees were covered with mud. That result in the isolation of the carbon from the carbon cycle. With the ongoing geologic process, the carbon was getting deeper into the ground, being exposed to higher temperatures and pressures, and eventually that resulted in the coal.

nuclear fission produces elements that are heavier than helium.

the elements that are used in nuclear fission and their products are much heavier than helium.