its option A (sarcoplasmic reticulum) because presence of Ca+2 ions in sarcoplasmic reticulum helps for the contraction of muscles (we have studied that as action potential passes ca+2 ions from the sarcoplasmic reticum will be released into cytoplasm and thus the troponin which covers the active sites of actin
move into cytoplasm and bind to ca+2 ions. This results in the exposure of active sites of actin and leads to contraction of muscle) .
Though bone is said to be made of calcium phosphate it is not said to be the reservoir of ca+2 ions.
as far as i know nucleus and tonoplast (layer around vacuole) are in no way associated with calcium.