ParentCo Purchased 100% of SubCo's Stock on January 1, 2014

ParentCo purchased 100% of SubCo's stock on January 1, 2014, and the companies have filed consolidated returns since then. Taxable income computations for the members include the following. Neither group member incurred any capital gain or loss transactions during these years, nor did they make any charitable contributions. The 2015 net operating loss:

A) may be carried back to offset SubCo's 2013 taxable income.
B) may be carried forward only and applied against group income if so elected by ParentCo.
C) cannot be carried back against 2013 SubCo income, as consolidated returns were not filed.
D) either a or b, but not both.