The force generated by a single muscle fiber can be increased by
a) increasing frequency of action potentials only.
b) increasing muscle length beyond optimal only.
c) increasing the asynchrony of recruitment only.
d) increasing frequency of action potentials and increasing muscle
length beyond optimal only.

A) increasing frequency of action potentials only.

Explanation:

If the muscle fibers begin the contraction with the sarcomere too elongated, the thick and thin filaments would be barely overlapping, forming few crossed bridges. On the contrary, if the sarcomere is too shortened so that the thick filaments reach the z-disks, the myosin is unable to find new fixation sites for cross-bridge formation and the tension decreases rapidly.

But to better understand how the contraction force increases, it should be noted that a single contraction does not represent the maximum force that a muscle fiber can develop. The force generated by the contraction of a muscle fiber is increased, increasing the frequency with which the action potentials of the muscle stimulate said fiber, that is to say that the nervous system controls part of this.

A potential muscular type action lasts between 1 and 3 ms. While muscle contraction can last 100 ms. If the repeated action potentials are separated by prolonged intervals of time, the muscle fiber has time to fully relax between the stimuli. If the action potentials continue to stimulate muscle fiber repeatedly at short intervals (high frequency), the relaxation between contractions decreases until the muscle fiber achieves a state of maximum contraction known as tetanus.

sodium i think the chemical element of atomic number 11, a soft silver-white reactive metal of the alkali metal group.