Lyophobic sol can be protected
a) by addition of oppositely charged sol
b) by addition of an electrolyte
c) by addition of lyophilic sol.
d) by boiling:

Options

A. by addition of oppositely charged sol
B. by addition of an electrolyte
C. by addition of lyophilic sol.
D. by boiling:

Explanation

To solve the question about how to protect a lyophobic sol, let's analyze each option step by step.

### Step 1: Understanding Lyophobic Sols
Lyophobic sols are colloidal solutions that do not have a strong affinity for the dispersion medium (usually water). They are sensitive to changes in their environment, such as temperature and the presence of electrolytes, which can lead to coagulation or precipitation.

**Hint**: Remember that lyophobic means "liquid-hating," indicating a weak interaction with the solvent.

### Step 2: Analyzing Each Option
1. **Addition of Oppositely Charged Sol**:
- When an oppositely charged sol is added to a lyophobic sol, it can lead to coagulation. This is because the opposite charges neutralize the charges on the lyophobic particles, causing them to aggregate and precipitate.
- **Conclusion**: This option is incorrect.

**Hint**: Think about charge interactions; opposites attract, leading to coagulation.

2. **Addition of an Electrolyte**:
- Electrolytes can also cause coagulation of lyophobic sols. They provide ions that neutralize the charge on the colloidal particles, leading to aggregation.
- **Conclusion**: This option is also incorrect.

**Hint**: Consider how electrolytes affect the stability of colloids by neutralizing charges.

3. **Addition of Lyophilic Sol**:
- Lyophilic sols are "liquid-loving" and have a strong affinity for the dispersion medium. When added to a lyophobic sol, they can form a protective layer around the lyophobic particles. This stabilizes the lyophobic sol and prevents it from coagulating.
- **Conclusion**: This option is correct.

**Hint**: Think about how a protective layer can shield the delicate particles from external influences.

4. **Boiling**:
- Boiling a lyophobic sol will increase the temperature, which can lead to coagulation due to the increased kinetic energy of the particles. This option does not protect the sol.
- **Conclusion**: This option is incorrect.

**Hint**: Remember that heating typically destabilizes colloids rather than stabilizing them.

### Final Answer
The correct answer is **c) by addition of lyophilic sol**. This method protects the lyophobic sol by forming a protective layer around it, preventing coagulation.