Which country became the last Dominion of the British Empire?

Dominions were the semi-independent polities under the British Crown that constituted the British Empire, beginning with Canadian Confederation in 1867. "Dominion status" was a constitutional term of art used to signify an independent Commonwealth realm; they included Canada, Australia, New Zealand, Newfoundland, South Africa, and the Irish Free State. India and Pakistan were also dominions for a short period of time, as was Ceylon (now Sri Lanka). The Balfour Declaration of 1926 recognised the Dominions as "autonomous Communities within the British Empire", and the 1931 Statute of Westminster confirmed their full legislative independence.

Ceylon, which, as a Crown colony, was originally promised "fully responsible status within the British Commonwealth of Nations", was the last country officially made a Dominion.

In 1972 it adopted a republican constitution to become the Free, Sovereign and Independent Republic of Sri Lanka. By a new constitution in 1978, it became the Democratic Socialist Republic of Sri Lanka.