Geologist discovered that the largest coal deposits were formed around 300 million years ago. how did this observation support the theory of plate tectonics?

Around that time, "those regions were located in tropical climates" that were "partially submerged" on the coasts.

Explanation:

300 million years ago from now, the mean sea level was higher than what it is today. Hence, the oceans had a larger spread on the continental land. The tectonic plate movements decreased the mean sea levels drastically and the ocean waters thus shrunk back far backward horizontally. This backward descend of the ocean waters exposed the coal deposits.