The IRS sets an interest rate each quarter that is applicable to underpayments by individuals. The rate for the third and fourth quarters of 2020 is 3%. You can see all interest rates listed by the National Association of Tax Professionals at https://bit.ly/3bVcCmw.
A mutual-fund distribution allocated to gains realized on the sale of fund portfolio assets. You report the distribution as long-term capital gain even if you held the fund shares short term.