Final answer:
The correct option in the final answer is a. The mitochondrial matrix is the site of DNA replication within mitochondria, where the mitochondrial DNA resides and the Krebs cycle occurs.
Step-by-step explanation:
The site of DNA replication within a mitochondrion is localized to the mitochondrial matrix. The mitochondrion is a double membrane-bound organelle that plays a critical role in cellular respiration. Its structure allows for distinct areas where various cellular processes occur. The inner membrane is folded into cristae to increase surface area for ATP synthesis and houses the electron transport chain. The intermembrane space lies between the inner and outer membranes.
Located inside the inner membrane is the mitochondrial matrix, a fluid-filled compartment containing metabolic enzymes, 70S ribosomes, and mitochondrial DNA. It is within the matrix that the mitochondrial DNA, which is different from the nuclear DNA, replicates. The mitochondrial matrix also plays host to the Krebs cycle, a series of biochemical reactions essential for cellular respiration.
Thus, when addressing the question of DNA replication in mitochondria, the correct option is the mitochondrial matrix, which is option 'a. Matrix'.