Which of the following is known as the store house of calcium ions?

Sarcoplasmic reticulum.

The correct answer is Option (3) - Sarcoplasmic reticulum.

The sarcoplasmic reticulum within muscle fibers serves as the storage site for calcium ions. When a neural signal reaches this junction, it prompts the release of a neurotransmitter (acetylcholine), triggering an action potential in the sarcolemma. This action potential propagates through the muscle fiber, leading to the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm. This initiation of calcium release sets in motion the mechanism for muscle contraction. Therefore, the sarcoplasmic reticulum in muscle fibers functions as the reservoir for calcium ions.