Calcium occurs in

Options

A. All body cells
B. Bones and teeth
C. Cells of respiratory system
D. 3% of mineral content.

Explanation

**Step-by-Step Solution:**

1. **Understanding Calcium as a Mineral:**
- Calcium is classified as a mineral and is the most abundant mineral in the human body. It plays a crucial role in various physiological functions.

2. **Calcium Distribution in the Body:**
- Approximately 99% of the body's calcium is stored in the bones and teeth. This makes bones and teeth the primary sites for calcium storage.

3. **Calcium in Other Body Cells:**
- While calcium is predominantly found in bones and teeth, a small amount (about 1%) is present in other body cells. This includes nerve cells and muscle cells, which require calcium for proper functioning.

4. **Evaluating the Options:**
- Option A: "All body cells" - This is incorrect as calcium is not present in all body cells.
- Option B: "Bones and teeth" - This is correct since the majority of calcium is stored here.
- Option C: "Cells of the respiratory system" - This is incorrect as calcium is not specifically concentrated in these cells.
- Option D: "3% of the mineral content" - This is also incorrect, as calcium constitutes about 99% of the mineral content in bones.

5. **Conclusion:**
- The correct answer to the question "Calcium occurs in" is **B: Bones and teeth**.