IRC § 1361(b)(3)
Termination of QSub Election Due to 80% Acquisition of QSub by C Corp
When the common parent of a consolidated group acquires 80 percent of a qualified subchapter S subsidiary, the QSub election terminates and the subsidiary joins the buyer’s consolidated group.
Treas. Reg. § 1.1361-5(a)(4), Example 5 • Qualified Subchapter S Subsidiaries