Final answer:
The sarcoplasmic reticulum sequesters Ca2+ inside muscle fibers, playing a key role in the muscle contraction and relaxation by regulating intracellular calcium levels.
Step-by-step explanation:
The organelle that sequesters Ca2+ inside muscle fibers is the sarcoplasmic reticulum (SR). The sarcoplasmic reticulum is a specialized type of endoplasmic reticulum found in muscle cells and is crucial for muscle functioning. It is responsible for storing, releasing, and retrieving calcium ions that are essential for triggering muscle contraction and relaxation.
During the process of muscle fiber contraction and relaxation, calcium ions are actively transported into the sarcoplasmic reticulum and are released into the sarcoplasm to interact with contractile proteins which result in muscle contraction. The sarcoplasmic reticulum's ability to sequester calcium is fundamental to the regulation of the intracellular calcium concentration in muscle cells. In summary, the correct answer to the question is C. The sarcoplasmic reticulum.