Final answer:
A muscle fiber exposed to a series of stimuli of increasing frequency will experience summation that can lead to complete sustained contraction, also known as complete tetanus, when the relaxation phase between individual twitches no longer occurs.
Step-by-step explanation:
When a muscle fiber is exposed to a series of stimuli of increasing frequency, it experiences a phenomenon known as summation, which leads to a cumulative build-up of muscle tension. If the frequency of stimulation is sufficiently high, the muscle fiber stops relaxing between twitches, leading to a continuous state of contraction referred to as complete tetanus. Therefore, the correct answer to the question is A. complete sustained contraction.
During incomplete tetanus, muscle contractions occur with a short relaxation phase between them. However, once the frequency of the stimuli surpasses a certain threshold, the relaxation phase disappears, yielding a full, continuous contraction. This physiological mechanism is essential for producing the forceful and sustained contractions needed for various bodily functions.