Final answer:
D. Inhibition of acetylcholine release at neuromuscular junctions. The inhibition of acetylcholine release at neuromuscular junctions decreases the likelihood of skeletal muscle contraction as it prevents the initiation of the excitation-contraction sequence.
Step-by-step explanation:
Among the options provided, the condition that will decrease the likelihood of skeletal muscle contraction is D. Inhibition of acetylcholine release at neuromuscular junctions. The release of acetylcholine (ACh) is a critical step in the initiation of a muscle contraction. Without the release of ACh, the muscle fiber will not depolarize, and the subsequent steps that lead to muscle contraction, including the release of calcium ions (Ca++) from the sarcoplasmic reticulum (SR), will not occur. This means that the troponin-tropomyosin complex will not be activated, and myosin will not bind to actin to form cross-bridges.
In contrast, increased calcium ion concentration in the cytoplasm would promote contraction, as would the activation of the troponin-tropomyosin complex which exposes the myosin-binding sites on actin filaments, enabling the myosin heads to bind and initiate contraction. The binding of ATP to myosin heads is also essential since it is needed for the cross-bridge cycle to continue, allowing muscles to maintain or regulate contraction.