Which of the following Pacific Islander countries is ruled by a constitutional monarchy?

Tonga is ruled by a constitutional monarchy, making it the correct answer among the given Pacific Islander countries. In this system, the country is governed by a monarch, in this case, King Tupou VI, who acts as the symbolic head of state, while the government is run by an elected parliament. This form of government provides a balance between the traditional monarchy and modern democratic principles, allowing for political stability and continuity in Tonga.