Final answer:
Large coal fields in Pennsylvania from the Carboniferous Period indicate a historically warmer climate, best explained by movements of tectonic plates which altered the region's climate by changing its position relative to the equator over millions of years.
Step-by-step explanation:
The large coal fields found in Pennsylvania, which date back to the Carboniferous Period, are evidence of a climate that was once much warmer in the northeastern United States. The change in climate over such a long period is best explained by the movements of tectonic plates. During the Carboniferous Period, around 360 to 290 million years ago, widespread forests of large plants, such as club mosses and horsetails, thrived. The decomposition of these forests under specific conditions led to the formation of massive deposits of carbon, which we now extract as coal.
The climate during this time was warmer and more conducive to such vast forests compared to the current climate of Pennsylvania. When considering the options given, movements of tectonic plates (A) are the most likely explanation for these significant climatic changes. These movements have shifted the continents over millions of years, altering their positions relative to the equator and changing the climate experienced in different regions. These shifts can explain why a region that was once tropical and supported lush forests would now have a different climate.