A. To wake people up in the morning?
B. To communicate between Île Saint-Paul (now Île-des-Soeurs) and the farm at Maison Saint-Gabriel?
C. To guide boats on the St. Lawrence River?
For the answer ...
This fog horn was used as a megaphone between the two farms, to call the boat to carry people across, among other things.
It should be noted that the level of ambient noise was very different at that time and that the sound carried over the water.