Final answer:
The sarcoplasmic reticulum (SR) in skeletal muscle cells stores the calcium ions necessary for muscle contraction. Upon stimulation, it releases these ions to initiate the contraction process.
Step-by-step explanation:
In skeletal muscle cells, the sarcoplasmic reticulum (SR) is the specialized organelle responsible for storing calcium ions (Ca2+) that are pivotal for initiating muscle contraction. When a muscle fiber is stimulated by an action potential, the SR releases calcium ions into the sarcoplasm. This increase in calcium concentration triggers the interaction between actin and myosin filaments within the sarcomere, leading to muscle contraction. The correct answer to which muscle cell structure stores calcium ions used to trigger contraction is d) Sarcoplasmic reticulum.