Final answer:
The opinion expressed by William Jennings Bryan on income tax was written into law by the 16th Amendment to the Constitution, which was ratified in 1913 to authorize federal income tax. The correct answer is option: (2) The 16th Amendment to the Constitution.
Step-by-step explanation:
The opinion expressed by William Jennings Bryan in his speech advocating for an income tax was ultimately written into law through the 16th Amendment to the Constitution. This constitutional amendment, ratified in 1913, authorized the federal government to impose a federal income tax.
The 16th Amendment was a response to the need for a steady revenue source for the government, beyond tariffs, and was established after the Supreme Court declared earlier federal income tax laws unconstitutional. The Progressive Era reforms and the need to make taxation more equitable led to the amendment's creation and its eventual inclusion in the law, solidifying the federal government's power to tax incomes.