The correct answer is "transmission electron microscope".
The transmission electron microscope can magnify the organelles of the cell clearly; as in light microscopes wherein most organelles cannot be visualized. TEM works by using a beam of electrons to pass through a specimen to make a clear image. Because of its superior magnification, mitochondrial defects can be visualized easily.
Attached is a sample picture of a transmission electron microscope image of a mitochondria.