Final answer:
The correct answer to the question is B) Sarcomere, which contracts upon neural stimulus at the neuromuscular junction, leading to muscle fiber contractions through the sliding filament theory.
Step-by-step explanation:
When stimulated by a neural signal, each sarcomere contracts, causing electrical signals. Thus, the correct answer is B) Sarcomere. Muscle contraction occurs at the neuromuscular junction, where a nerve impulse prompts the release of the neurotransmitter acetylcholine (ACh), leading to the depolarization of the muscle fiber. This event triggers an action potential that spreads through the sarcolemma and prompts the release of calcium ions from the sarcoplasmic reticulum, enabling the muscle fiber to contract. The sliding filament theory explains this process, with myosin and actin filaments sliding past each other, shortening the sarcomeres and causing the fiber to contract.