Final answer:
Mitochondria are membrane-enclosed organelles found in eukaryotic cells that produce ATP through cellular respiration and serve as the cell's energy source. So the correct option is B.
Step-by-step explanation:
A mitochondrion (mitochondria, plural), is a membrane-enclosed organelle that is found in most eukaryotic cells. Mitochondria are called the "power plants" of the cell because they are the sites of cellular respiration, where they use energy from organic compounds to make ATP (adenosine triphosphate). ATP is the cell's energy source that is used for such things as movement and cell division.
Some ATP is made in the cytosol of the cell, but most of it is made inside mitochondria. The number of mitochondria in a cell depends on the cell's energy needs. For example, active human muscle cells may have thousands of mitochondria, while less active red blood cells do not have any.