Canada is a member of the commonwealth. this membership is a result of canada having once been a part of which empire?

Initially, northerners did not want missouri to be included in the union because 1.) maine would not be allowed to enter the union as a free state 2.) missouri wanted to enter the union as a free state 3.) missouri wanted to enter the union as a slave state 4.) maine would not be allowed to enter the union as a slave state