1. Oxidation of isocitrate to α- ketoglutarate- Isocitrate gives off a pair of hydrogen atoms and a molecule of carbon dioxide and becomes 5-carbon α-ketoglutarate. The pair of hydrogen atoms gives two electrons and one H⁺ to NAD⁺ forming NADH + H⁺. The reaction is catalyzed by enzyme isocitrate dehydrogenase in the presence of manganese ions. 

2. Oxidation of α-ketoglutarate to succinyl coenzyme A-In this reaction, coenzyme A reacts with α-ketoglutarate, forming 4-carbon succinyl coenzyme A and releasing carbon dioxide and a pair of hydrogen atoms. The reaction is catalyzed by α-ketoglutarate dehydrogenase complex enzyme. The pair of hydrogen atoms passes two electrons and one H⁺ to NAD⁺ forming NADH + H⁺.

Thus, the correct answer is option A.