Citric acid is produced by -

Options

A. Acetobacter aceti (a bacterium)
B. Yeast (a fungus)
C. Aspergillus niger (a fungus)
D. Streptococcus (a bacterium)

Answer

**Step-by-Step Solution:**

1. **Identify the Microorganism**: The question asks about the production of citric acid. The key microorganism responsible for this production is **Aspergillus niger**, which is a type of fungus.

2. **Understand the Process**: Aspergillus niger utilizes carbohydrates, particularly starch, as a source of energy. Through a process called fermentation, it converts these carbohydrates into citric acid.

3. **Distinguish from Other Microorganisms**: While other microorganisms like **Acetobacter** and **yeast** are mentioned, they are not responsible for citric acid production. Acetobacter is involved in converting ethanol to acetic acid (vinegar), and yeast is primarily used in baking and fermentation processes to produce carbon dioxide.

4. **Applications of Citric Acid**: Citric acid produced by Aspergillus niger is widely used in the food industry as a preservative and flavoring agent, as well as in the pharmaceutical industry for various applications.

5. **Conclusion**: Therefore, the correct answer to the question "Citric acid is produced by" is **Aspergillus niger**.

---