Answer: C "an unobstructed passage to the Atlantic"
The excerpt from the informational document describes several advantages that Britain had for manufacturing, including its excellent commercial position as a middle point between the north and south of Europe, its insular situation, and its command of the seas. Of these advantages, the one that best supports the thesis that Britain's geographic advantages supported trade is "an unobstructed passage to the Atlantic." This detail suggests that Britain's location gave it easy access to the Atlantic Ocean, which was a major trade route at the time. This access would have allowed British manufacturers to easily export their goods to other countries, contributing to the country's economic success.