Final answer:
REITs must derive at least 75% of their gross income from sources related to real estate, most notably rental income, to comply with IRS regulations.
Step-by-step explanation:
REITs, or Real Estate Investment Trusts, must meet certain income criteria to comply with IRS regulations. Specifically, they must derive at least 75% of their gross income from certain real estate-related sources. Rental income, interest on mortgages financing real property or on real property-related debt instruments, and gains from the sale or other disposition of real property, including any gains from foreclosure property, all count towards this requirement. Therefore, out of the options provided, rental income is a source that REITs must derive at least 75% of their gross income from according to IRS guidelines.