…....... should be present in the water form metamorphosis of tadpoles.

Options

A. Chlorine
B. Carbon
C. Sulphur
D. Iodine

Explanation

**Step-by-Step Solution:**

1. **Understanding Metamorphosis**: The process of metamorphosis in tadpoles involves their transformation into adult frogs. This process is crucial for their development and growth.

2. **Environment of Tadpoles**: Tadpoles live in water during their early life stages. The conditions and substances present in the water can significantly affect their growth and metamorphosis.

3. **Identifying Essential Elements**: To determine what should be present in the water for the metamorphosis of tadpoles, we need to consider the substances that are vital for their development.

4. **Evaluating Options**: The options provided are chlorine, carbon, sulphur, and iodine. We need to analyze which of these is essential for the metamorphosis process.

5. **Role of Iodine**: Among the options, iodine is known to be crucial for the production of the hormone thyroxine. Thyroxine plays a significant role in regulating metabolism and is essential for the growth and development of tadpoles into adult frogs.

6. **Conclusion**: Therefore, iodine should be present in the water for the metamorphosis of tadpoles to occur effectively.

**Final Answer**: Iodine should be present in the water for the metamorphosis of tadpoles.

---