Passage: The US Exploring Expedition visited in 1840.In 1845, the ambitious young warrior, strategist, and orator Tāufaʻāhau united Tonga into a kingdom. He held the chiefly title of Tuʻi Kanokupolu, but had been baptised by Methodist missionaries with the name Siaosi ("George") in 1831. In 1875, with the help of missionary Shirley Waldemar Baker, he declared Tonga a constitutional monarchy; formally adopted the western royal style; emancipated the "serfs"; enshrined a code of law, land tenure, and freedom of the press; and limited the power of the chiefs.

Question: In what year did Taufa'ahau, the Tu'i Kanokupolu, declare Tonga a constitutional monarchy?
1875