The correct answer to this open question is the following.
You forgot to include the options of the question and the map for reference. However, doing some research, we can say the following.
What served as much of the dividing line between the British colonies and the French colonies was "The Appalachian Mountains served as most of the boundary."
We are talking about the North American territory in colonial America, 1700. In that time, Great Britain had colonized the East Coast of the territory to the Atlantic Ocean. The Appalachian Mountains served as most of the boundary. So the Western part of the territory .what later was known to the Louisiana territory- was controlled by the French. The Spanish crown also controlled some territories in that time, as was the case of the Florida Peninsula and the New Hispania.