d. propagation of an action potential along the sarcolemma and T tubules
Explanation:
Excitation-contraction coupling occurs in the body as an electrical signal to the muscles, which must capture this signal and perform muscle contraction.
After this signal is emitted, an action potential is generated that will act on the neuromuscular junction and spread quickly and coordinated by the sarcolemma, reaching the T tubules.
In the T tubules, receptors of dihydropyridine receptors are located, which will continue the process, until muscle contraction is performed.
Excitation-contraction coupling refers to the series of events that begins with the excitation of the sarcolemma in response to stimulation by a neurotransmitter, and results in the release of calcium ions from the sarcoplasmic reticulum.
Hence, option D is the 1st process that starts the excitation-contraction coupling.
Chloroplast are an organelle in the plant cell that converts sunlight into energy and uses it for the growth of it. It stores extra water for later use.
