Regarding the statute of limitations on additional assessments of tax by the IRS, determine the applicable period in each of the following situations. Assume a calendar year individual with no fraud or substantial omission involved.

a. The income tax return for 2020 was filed on February 19, 2021.

b. The income tax return for 2020 was filed on June 25, 2021.

c. The income tax return for 2020 was prepared on April 4, 2021, but was never filed. Through some misunderstanding between the preparer and the taxpayer, each expected the other to file the return.

d. The income tax return for 2020 was never filed because the taxpayer thought no additional tax was due.