The composition of sperm can be divided into three parts: the head, the middle piece, and the tail. The middle piece of the sperm is the thickest part and contains various organelles. The correct answer to the given question is option 'B' - mitochondria.

Mitochondria in the Middle Piece of Sperm:
Mitochondria are the powerhouses of the cell, responsible for producing energy in the form of ATP. In the middle piece of the sperm, there are numerous mitochondria arranged in a spiral pattern around the microtubules of the tail. These mitochondria provide energy for the sperm to move and swim towards the egg for fertilization. The amount and quality of mitochondria in the sperm can affect its motility and fertility potential.

Other Organelles in the Middle Piece of Sperm:
Apart from mitochondria, the middle piece of the sperm also contains other organelles such as:

- Centriole: It is responsible for the formation of the microtubules that make up the tail of the sperm.

- Acrosome: A cap-like structure at the anterior end of the sperm head that contains enzymes to help the sperm penetrate the outer layers of the egg during fertilization.

- Golgi Apparatus: It is involved in the formation and maturation of the acrosome.

Conclusion:
In conclusion, the middle piece of the sperm contains several organelles, including mitochondria, centriole, acrosome, and Golgi apparatus. However, the predominant organelle in the middle piece is mitochondria, which provides energy for the sperm's movement and motility.