Final answer:
The sarcoplasmic reticulum is responsible for the release and storage of calcium ions in muscle cells, which triggers muscle contraction when an action potential is generated.
Step-by-step explanation:
The structure responsible for the release of calcium ions in muscle cells is the sarcoplasmic reticulum. This specialized form of the endoplasmic reticulum acts as a storage site for calcium ions. During the process of muscle contraction, when an action potential reaches the muscle fiber, it travels down the T tubule and prompts the sarcoplasmic reticulum to release the stored Ca²⁺ ions into the sarcoplasm. This increase in calcium ion concentration initiates the series of reactions that lead to muscle contraction.
Thus, the correct answer to the question, 'Which of the following structures is responsible for the release of calcium ions?' is D. Sarcoplasmic reticulum. Additionally, when referring to the storage of calcium ions, the answer is D. sarcoplasmic reticulum as indicated in multiple choice options provided in the question context.