Final answer:
a) Calcium ions; troponin. The sarcoplasmic reticulum stores calcium ions in muscle cells and releases them for binding to troponin during muscle contraction.
Step-by-step explanation:
The sarcoplasmic reticulum is responsible for storing calcium ions in muscle cells. When a muscle is relaxed, the sarcoplasmic reticulum stores calcium ions. During contraction, the sarcoplasmic reticulum releases calcium ions to bind to troponin, which is involved in regulating skeletal muscle contraction.