The correct answer is Finland.
Finland holds the distinction of being the first country in the world to introduce a carbon tax, doing so in 1990. This tax was designed to put a price on the carbon dioxide emissions released from burning fossil fuels, with the goal of discouraging heavy use of coal, oil, and natural gas and encouraging a shift toward cleaner energy sources, as part of the country's response to concerns about global warming.
Several other Nordic countries such as Sweden, Norway, and Denmark followed Finland's lead and introduced their own carbon taxes in the years shortly after 1990. Japan introduced a carbon tax much later, in 2012. Germany has used other mechanisms like energy taxes and, later, participation in the EU Emissions Trading System, rather than being first with a dedicated carbon tax. Australia introduced a carbon pricing mechanism only in 2012, and it was subsequently repealed in 2014.
Because Finland was the pioneer in 1990, it is correctly identified as the first country to propose a carbon tax.