The state of Green imposes a tax on the "income" of each of its residents. As defined in the taxing statute, "income" includes the fair rental value of the use of any automobile provided by the taxpayer's employer for the taxpayer's personal use. The federal government supplies automobiles to some of its employees who are resident in Green so that they may perform their jobs properly. A federal government employee supplied with an automobile for this purpose may also use it for the employee's own personal business. Assume there is no federal legislation on this subject. May the state of Green collect this tax on the fair rental value of the personal use of the automobiles furnished by the federal government to these employees? 
No, because such a tax would be a tax on the United States.  No, because such a tax would be a tax upon activities performed on behalf of the United States, since the automobiles are primarily used by these federal employees in the discharge of their official duties.  Yes, because the tax is imposed on the employees rather than on the United States, and the tax does not discriminate against persons who are employed by the United States.  Yes, because an exemption from such state taxes for federal employees would be a denial to others of the equal protection of the laws.