Final answer:
The muscle cell in the thigh of a long-distance runner would have the most mitochondria because these cells require a lot of energy to sustain prolonged contraction.
Step-by-step explanation:
The cell that would have the most mitochondria is c. muscle cell in the thigh of a long-distance runner. Mitochondria are responsible for producing adenosine triphosphate (ATP), the main energy-carrying molecule of the cell, through a process called cellular respiration.
Since muscle cells, especially those in long-distance runners, have high energy demands to sustain contraction over long periods, they require a large number of mitochondria to meet their energy needs.