Answer:
The correct answer is D) Only cells containing mitochondria can reproduce.
Step-by-step explanation:
Cell theory mainly states that every organism present on this earth are made up of cell and the cell is the basic and smallest unit from which life is made up of. It also states that all the new cell that comes into existence are evolved from the old(pre-existing) cells.
Therefore any cell that contain or do not contain mitochondria can reproduce because new cells come from an old cell not from mitochondria, for example, a bacterial cell does not have mitochondria but they reproduce by binary fission.
Therefore any cell which is living can reproduce so the principle which is not currently understood is D) Only cells containing mitochondria can reproduce.