D. 70-120 m/s  

A-alpha fibers are the largest, heavily myelinated somatic motor/Ia proprioceptive axons, and microelectrode recordings show their single-fiber conduction velocities can reach the high double-digit to low triple-digit range (up to ~120 m/s) [Hoffer et al., 1981, PMID 7300428]; routine human nerve studies still measure the fastest motor or sensory components around 70–80 m/s in large myelinated fibers [Macefield et al., 1989, PMID 2597994]. Hence the accepted textbook range is 70–120 m/s.