Alum is used by dyers of cloth :

Options

A. for fire-proofing fabrics
B. as first aid for cuts
C. for softening hard water
D. as mordant

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question states that alum is used by dyers of cloth. We need to determine the purpose of alum in this context.

2. **Identifying the Use of Alum**: Alum is commonly known for its various applications, including its use in dyeing processes. It is important to identify what role alum plays in dyeing fabrics.

3. **Defining Mordant**: A mordant is a substance that helps fix dyes onto fabrics. It does this by forming a coordination complex with the dye, which helps it adhere to the fabric.

4. **Explaining the Mechanism**: When alum is used as a mordant, it forms coordination complexes with the dye molecules. These complexes then attach to the fabric, ensuring that the dye does not wash out easily and remains vibrant.

5. **Conclusion**: Therefore, the correct answer to the question is that alum is used as a mordant by dyers of cloth.

**Final Answer**: Alum is used by dyers of cloth as a mordant to set dyes on fabrics.

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