Final answer:
The Confederation of Canada was formed in 1867 by Nova Scotia, New Brunswick, Ontario, and Quebec, united under the British North American Act, with Sir John A. Macdonald as its first Prime Minister.
Step-by-step explanation:
The four provinces that first joined to form the Confederation of Canada in 1867 were Nova Scotia, New Brunswick, and the Province of Canada which was later divided into Ontario and Quebec. These provinces united under the British North American Act for several reasons, including national defense and the construction of a transcontinental railroad. The creation of the Dominion of Canada marked an important step in Canadian history, with Queen Victoria as the head of state and Sir John A. Macdonald becoming the nation's first Prime Minister. This pivotal event established a self-governing entity within the broader British Empire and set the stage for future expansions and the development of the Canadian political landscape.