Final answer:
For skeletal muscle fibers to relax, both the destruction of ACh in the synaptic cleft by acetylcholinesterase and the removal of Ca++ from the sarcoplasm are required.
Step-by-step explanation:
During the relaxation of a skeletal muscle fiber, several processes must occur. After a contraction is initiated by the neurotransmitter acetylcholine (ACh) at the neuromuscular junction (NMJ), the muscle fiber needs to repolarize, which leads to the closing of the calcium channels in the sarcoplasmic reticulum (SR). Calcium ions (Ca++) are then actively pumped back into the SR by ATP-driven pumps, which helps unbind them from the actin-binding sites and allows tropomyosin to cover these sites again, preventing myosin from binding to actin. This process leads to the muscle fibers relaxing and the muscle returning to its resting state. Concurrently, acetylcholinesterase breaks down and removes ACh from the synaptic cleft, preventing continuous stimulation of muscle contraction.
Therefore, the correct answer to this question is e, both the destruction of ACh in the synaptic cleft and the removal of Ca++ from the sarcoplasm are necessary for a muscle fiber to relax after contraction.