Final answer:
A taxpayer should include income from salary, business, and investments when filing their tax return, as all these are forms of taxable income that must be reported to the federal government.
Step-by-step explanation:
A taxpayer should include all the following types of income when filing their tax return: 1) Income from salary, 2) Income from business, and 3) Income from investments. It's essential to report all sources of income to adhere to tax laws and ensure accurate calculation of taxes owed. The federal government taxes these various sources of income, and they are critical components of one's taxable income. Understanding these categories can be crucial for personal finance and financial planning.