Severe internal grooving corrosion is found at the bottom of a 4-inch (101.

API RP 571 explains under Condensate Corrosion (CO₂ Corrosion):

“In condensate systems, carbon dioxide dissolves in the water forming carbonic acid, which leads to localized corrosion, including grooving and under-deposit attack, especially at low points like the bottom of horizontal piping.”

“This is most commonly observed in steam condensate return lines, and the damage is localized due to stratification.”

This matches the scenario described, where internal grooving at the bottom of steam condensate piping is characteristic of CO₂-induced corrosion. Hence, option A is correct.