The chemical substance which acts as emulsifier is:

Options

A. Phosphoric acid
B. Fatty acid
C. Bile acids
D. Mineral acids `(HCl)`

Answer

To solve the question "The chemical substance which acts as emulsifier is:", we will analyze the options provided and determine which one qualifies as an emulsifier based on its chemical properties.

### Step-by-Step Solution:

1. **Understanding Emulsifiers**:
- Emulsifiers are substances that help to stabilize emulsions, which are mixtures of two immiscible liquids (like oil and water).
- They are typically amphipathic molecules, meaning they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts.

2. **Analyzing the Options**:
- **Phosphoric Acid**: This is a strong acid and does not have the amphipathic nature required for emulsification. It is not suitable as an emulsifier.
- **Fatty Acid**: Fatty acids are amphipathic molecules; they have a hydrophilic carboxylic acid group and a long hydrophobic hydrocarbon chain. They can act as emulsifiers.
- **Bile Acids**: Bile acids are also amphipathic molecules, containing both hydrophilic and hydrophobic regions. They are known to act as emulsifiers in the digestive system, helping to emulsify fats.
- **Mineral Acids (HCl)**: Hydrochloric acid is a strong acid and does not possess the properties of an emulsifier. It does not have the amphipathic structure needed.

3. **Conclusion**:
- Among the options, bile acids are the most recognized emulsifiers due to their amphipathic nature. While fatty acids can also act as emulsifiers, the question asks for the best option, which is bile acids.

### Final Answer:
The chemical substance which acts as an emulsifier is **bile acids** (Option C).