Choice "A" is correct. For a 30% understatement of gross income (anything over 25%), the statute of limitations is 6 years.
Choice "b" is incorrect. For a 25% (or less) understatement of gross income, the statute of limitations is 3 years.
Choice "d" is incorrect. There is no 7-year statute of limitations for an understatement of gross income.
Choice "c" is incorrect. The statute of limitations is unlimited for fraud and filing false returns, but not for understatements of income. There is no fraud in this question because the omission was inadvertent.
