….....is present in cysteine and methionine and is the main constituent of several coenzymes .

Options

A. Magnesium
B. Sulphur.
C. Potassium
D. Phosphorus

Answer

To solve the question, we need to identify the substance that is present in cysteine and methionine and is a main constituent of several coenzymes.

### Step-by-Step Solution:

1. **Identify the Amino Acids**:
- The question mentions two amino acids: cysteine and methionine.
- Both of these amino acids contain sulfur in their structure.

2. **Analyze the Role of Sulfur**:
- Sulfur is an essential element that plays a significant role in the structure of certain amino acids and is crucial for various biochemical processes.
- It is also a key component of several coenzymes.

3. **Consider Other Options**:
- The question lists other elements: magnesium, potassium, and phosphorus.
- While magnesium is important for chlorophyll and other functions, it is not present in cysteine and methionine.
- Potassium and phosphorus also do not fit the criteria since they are not components of these specific amino acids.

4. **Conclusion**:
- Since sulfur is the only element that is present in both cysteine and methionine and is also a main constituent of several coenzymes, the correct answer to fill in the blank is "sulfur".

### Final Answer:
**Sulfur** is present in cysteine and methionine and is the main constituent of several coenzymes.

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