Stream of consciousness rationale: To answer the above question, the relevant sentence is: Papua New Guinea became self-governing on 1 December 1973 and achieved independence on 16 September 1975.
The question and answer pair are described below.
Q: Elections in 1972 resulted in the formation of a ministry headed by Chief Minister Michael Somare, who pledged to lead the country to self-government and then to independence. Papua New Guinea became self-governing on 1 December 1973 and achieved independence on 16 September 1975. The country joined the United Nations (UN) on 10 October 1975 by way of Security Council Resolution 375 and General Assembly resolution 3368. The 1977 national elections confirmed Michael Somare as Prime Minister at the head of a coalition led by the Pangu Party. However, his government lost a vote of confidence in 1980 and was replaced by a new cabinet headed by Sir Julius Chan as prime minister. The 1982 elections increased Pangu's plurality, and parliament again chose Somare as prime minister. In November 1985, the Somare government lost another vote of no confidence, and the parliamentary majority elected Paias Wingti, at the head of a five-party coalition, as prime minister. A coalition, headed by Wingti, was victorious in very close elections in July 1987. In July 1988, a no-confidence vote toppled Wingti and brought to power Rabbie Namaliu, who a few weeks earlier had replaced Somare as leader of the Pangu Party.

Answer this question based on the passage: when did papua new guinea become an independent country?
A: 16 September 1975