The right answer is the Krebs cycle
In prokaryotes, the Krebs cycle occurs in the cytoplasm because they do not have mitochondria.
In eukaryotes, it occurs in the mitochondrial matrix (meaning that eukaryotes without mitochondria cannot breathe). Pyruvate (the first participant in the Krebs cycle) produced by glycolysis in the cytoplasm can, however, freely enter the mitochondria, both membranes being completely permeable to it.
