Final answer:
The number often referred to as an Employer Identification Number, required by all entities except sole proprietorships and LLCs with no employees is the Federal Employer Identification Number (FEIN). The Social Security tax, being a flat rate up to a certain income level, is regressive.
Step-by-step explanation:
The number often referred to as an Employer Identification Number which all entities require except sole proprietorships and LLCs with no employees is d) Federal Employer Identification Number (FEIN). This number is used by the Internal Revenue Service (IRS) to identify business entities for tax purposes. Sole proprietorships without any employees can use the owner's Social Security Number (SSN) instead of an FEIN.
If an individual owns a corporation and is the only employee, they have to pay various types of federal tax, including income tax, employment tax, unemployment compensation, social security insurance, and possibly excise tax. For a self-employed individual with an unincorporated business, they would pay self-employment tax (which covers both Social Security and Medicare taxes), income tax, and possibly additional taxes depending on the state regulations.
The Social Security tax is 6.2% on employees' earned income below $113,000. This tax is considered to be regressive, because it does not scale with income; all earnings up to the threshold are taxed at the same rate, meaning it takes a larger percentage of income from lower earners compared to higher earners.