Mark has come to your office for help in filing his $2023$ federal income tax return. He gives you the following information:

Mark is single and works for IBM. Mark received a salary of $$125,000$ in $2023,$ as reported on his W$-2.$ Mark had $$4,000$ of federal income tax withheld from his paychecks during the year.

Mark bought and sold various stocks in $2023.$ He had $$4,000$ in capital gains from the sales and $$8,000$ in capital losses from sales.

Next, assume that you filed the tax return based on the information above on April $1,2024.$ In May of $2024$ you discover that Mark did not mention to you that he received other taxable income of $$35,000$ in $2023.$ This was a one$-$time transaction and you are not sure if Mark intentionally omitted the information or if he made an honest mistake since it was a one$-$time transaction. Mark has asked you how long the IRS has to audit his tax return.

Please explain to Mark what the last day the IRS can audit his return and seek to impose additional tax is$.$ Be sure to fully explain your reasoning. IRC $6501$ will be critical to your analysis.