Gold number is a measure of the protective power of a lyophilic sol. It is the amount of a protective colloid in milligrams required to prevent the coagulation of 10 ml of a gold sol upon the addition of 1 ml of 10% NaCl solution.

The correct answer to the question is option 'C', which states that the lower the gold number, the greater is the protecting power of the sol. This means that a sol with a lower gold number requires a smaller amount of protective colloid to prevent coagulation, indicating a higher efficiency in protecting the sol from coagulation.

To understand why this is the case, let's discuss the concept of peptization and the role of protective colloids in stabilizing lyophilic sols.

1. Peptization:
Peptization is the process of converting a precipitate or a coagulated sol into a stable colloidal sol. When a precipitate or coagulated sol is formed, the particles come together and aggregate, leading to the formation of larger particles that are visible to the naked eye. Peptization involves breaking down these aggregates and dispersing the particles evenly in the medium to form a stable colloidal sol.

2. Protective colloids:
Protective colloids are substances that can prevent the coagulation or precipitation of a lyophilic sol. They work by adsorbing onto the surface of the sol particles, creating a protective layer that prevents the particles from coming into close contact and aggregating. This adsorption stabilizes the sol and prevents coagulation.

Now, let's discuss why a lower gold number indicates greater protecting power.

- The gold number is a measure of the protecting power of a sol. A lower gold number indicates that a smaller amount of protective colloid is required to prevent coagulation.
- This implies that the protective colloid is more efficient in preventing coagulation because it can stabilize the sol with a smaller amount.
- Therefore, a sol with a lower gold number has a higher protecting power as it requires less protective colloid to maintain stability.
- On the other hand, a sol with a higher gold number would require a larger amount of protective colloid to prevent coagulation, indicating a lower protecting power.
- In summary, the lower the gold number, the greater is the protecting power of the sol.

In conclusion, the correct answer to the given question is option 'C'. A lower gold number indicates a greater protecting power of a lyophilic sol as it requires a smaller amount of protective colloid to prevent coagulation.