Final answer:
The term recruitment refers to the increase in the number of motor units activated in a muscle as a result of more intense stimulation, allowing for gradation in muscle contraction strength. The correct option is B) recruitment.
Step-by-step explanation:
The increase in the number of motor units activated as a result of more intense stimulation is called recruitment. As the central nervous system requires more muscle force, larger motor units with high-threshold motor neurons are recruited to activate larger muscle fibers.
This process of recruitment allows skeletal muscles to produce a range of contraction forces, from the very slight to the exceptionally powerful, depending on the number of motor units activated.
When necessary, the maximum number of motor units can be recruited simultaneously for maximum contraction force, although this state cannot be maintained for long due to energy expenditure and the need to prevent muscle fatigue.