An income tax is imposed annually at the rate of five percent on the South Carolina taxable income of an organization described in Internal Revenue Code Sections 501 through 528 (Exempt Organizations) and 1381 (Cooperatives) as computed under Internal Revenue Code Sections 501(b) (unrelated business income), 1382 and 1383 (taxation of cooperatives) with the modifications provided in Article 9 of this chapter and subject to allocation and apportionment as provided in Article 17 of this chapter.