In a muscle fibre `Ca^(++)` is stored in `:-`

Explanation

**Step-by-Step Solution:**

1. **Identify the Structure**: In muscle fibers, calcium ions (Ca²⁺) are primarily stored in a specific organelle known as the sarcoplasmic reticulum (SR).

2. **Understand the Type of Reticulum**: The sarcoplasmic reticulum is a specialized form of smooth endoplasmic reticulum (SER). This means it does not have ribosomes on its surface, which distinguishes it from rough endoplasmic reticulum.

3. **Membrane Characteristics**: The sarcoplasmic reticulum is a single membrane-bound structure, which means it is enclosed by a single lipid bilayer.

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