Final answer:
Slow oxidative (SO) muscle fibers have the highest level of mitochondria and can sustain muscle activity without fatiguing for long periods of time.
Step-by-step explanation:
The muscle fibers with the highest level of mitochondria are the slow oxidative (SO) fibers. These fibers contract relatively slowly, but they have a large number of mitochondria which allows them to produce a large amount of ATP. The mitochondria in SO fibers use aerobic respiration to produce ATP, which can sustain muscle activity without fatiguing for long periods of time.