(a) Water's-edge reporting. —
Absent an election under subsection (b) of this section to report based upon a worldwide unitary combined reporting basis, taxpayer members of a unitary group shall determine each of their apportioned shares of the net business income or loss of the combined group on a water's-edge unitary combined reporting basis. In determining tax under this article and article twenty-three of this chapter on a water's-edge unitary combined reporting basis, taxpayer members shall take into account all or a portion of the income and apportionment factors of only the following members otherwise included in the combined group pursuant to section thirteen-a of this article:
(1) The entire income and apportionment factors of any member incorporated in the United States or formed under the laws of any state, the District of Columbia or any territory or possession of the United States;