Can you tell me if a fund like GBTC, Grayscale Bitcoin Trust, should be listed as a "Mutual Fund, Index Fund, or ETF" or as a stock?

When categorizing GBTC, Grayscale Bitcoin Trust, from short term capital gains and losses, I can select  the choice: "Mutual Fund, Index Fund, or ETF," or a categorize it as stock. Do you know which one I should select?