Who was the Prime Minister of Canada in 1968?

Nobody was elected Canada's prime minister in 1968, or at any other time. Each Canadian Prime Ministers is appointed by the Governor General of Canada, on the basis that she or he has the support, or "confidence" of a majority of elected Members of Parliament.

Pierre Elliot Trudeau was appointed as Prime Minister on April 20, 1968, prior to the election, because he was the individual most likely to hold the confidence of a majority of elected MPs at that time. He remained Prime Minister after the election in 1968, because he was supported by most of the Members of Parliament elected in that election.