Final answer:
During the Carboniferous Period, North America was joined to Africa as part of the supercontinent Pangaea, which was the time when the coal deposits of the Appalachian Mountain region began to form.
Step-by-step explanation:
During the time the swamp plants that eventually became the coal deposits of the Appalachian Mountain region were growing in a tropical environment, North America was part of a supercontinent. This supercontinent was Pangaea, which included all major land masses. Specifically, when considering the options provided, the correct answer is that North America was joined to Africa as part of Pangaea. This was the time of the Carboniferous Period, approximately 360-290 million years ago, when coal-forming forests flourished and the Appalachian Mountains began to form from collisions with other landmasses, including the one with Gondwana that added Africa to the conglomeration.