Answer: The correct answer for the fill in the blank is "The mitochondria".
A typical sperm consists of Head, neck, midpiece, and tail or flagellum.
Midpiece region of the sperm contains Mitochondria, which are densely packed in it. They provide energy in the form of ATP (adenosine triphosphate) that pushes the sperm cell in a particular direction.
On the contrary, flagellum executes movement in the sperm and nucleus (located in head) possesses chromatin fibres.
Thus, mitochondria is the right answer.