The right answer is A.
The Krebs cycle or citrate cycle occurs in mitochondria in eukaryotes. It comprises eight enzymatic reactions that can be decomposed into simple reactions. This final step of the oxidative catabolism of carbohydrates, fatty acids and amino acids ensures most of the energy needs of the cell through the formation of reduced coenzymes which will be reoxidized in the respiratory chain.
The respiratory chain (or electron transport chain) is a set of 4 enzymatic complexes, ubiquinone and cytochrome c, in the inner membrane of mitochondria. They are electron carriers and for some protons (from NADH and FADH2) which represent the final stage of energy production during the oxidation process of energy molecules.
A mitochondrial cristae is a fold of the inner mitochondrial membrane in which a large number of membrane proteins such as cytochromes and ATP synthase are concentrated.