Final answer:
Signal Hill is the historical site that served as a strategic military vantage point in the 1700s and is now recognized as a National Historical Site of Canada in the Atlantic region.
Step-by-step explanation:
The location that has acted as a strategic defensive site since the 1700s and is now a National Historical Site of Canada in the Atlantic region of Canada is C. Signal Hill. Signal Hill is significant due to its historical military and communications roles. It overlooks the city of St. John's in Newfoundland and Labrador. Historically, Signal Hill was strategic because it provided a high vantage point over the harbor and the Atlantic Ocean, allowing for effective defense against potential threats. During the 1700s, it played a key role in various military engagements, most notably the Battle of Signal Hill in 1762. Today, it is recognized as a place of historical importance and has been commemorated as a National Historical Site.