Final answer:
The federal government first adopted an income tax in 1861 to fund Civil War expenses. The 16th Amendment, ratified in 1913, established the legal basis for a permanent federal income tax. This system of taxation has grown and evolved into the predominant form of revenue for the U.S. government. Therefore, the correct option is 4.
Step-by-step explanation:
The first federal income tax was adopted in 1861 when the Union Congress needed additional funds to support the Civil War effort. Initially, the Confederate Congress enacted a direct tax on personal property, and the Union Congress enacted the nation's first income tax and also raised tariff rates to increase revenue. Later, the 16th Amendment was ratified in 1913, authorizing the federal government to impose a legal income tax without apportioning it among the states or basing it on the U.S. Census. This amendment followed the early attempts by the government to impose an income tax and address the political challenges that arose from taxing personal property and incomes, leading to a more formal and constitutional approach to taxation. It was after this amendment that income tax became a more permanent and significant part of federal government revenue, evolving over time to include more taxpayers and encompassing to today's tax policies where filing and payment are due annually on April 15.