Final answer:
The main island of Canada's second-largest metropolis is Montreal Island, which is located in the province of Quebec and serves as a significant French-speaking population center.
Step-by-step explanation:
The main island of Canada's second-largest metropolis is A) Montreal Island. Montreal is the largest city in the province of Quebec and has developed substantially since the mid-19th century, mainly due to its strategic location controlling access through the St. Lawrence River and the Great Lakes. This metropolitan area, with over 4 million residents, is significantly French-speaking and is a major center with diversified industries, including manufacturing and tourism.