In single kreb's cycle, decarboxylation takes place at following two steps: (1) At 4th step, where isocitrate gives off a pair of H-atoms (oxidation) and a molecule of CO2 (decarboxylation) and becomes 5−Cα-Ketoglutarate. (2) 5th step-where coenzyme-A reacts with α Ketoglutarate forming 4-C SuccinylCoenzyme A and releasing CO2 and a pair hydrogen atoms. The question is specifically asking for isocitrate