The gold number range of some of the lyophilic colloids is given below:
A: 0.005–0.01, B: 0.15–0.25, C: 0.04–1.0, D: 15–25
Which among these can be used as a better protective colloid?

The correct option is A.

To understand why A is the correct choice, let us look at the concept of the gold number and its relationship with the protective power of lyophilic colloids.

1. Definition of Gold Number:
The gold number is defined as the minimum amount of a protective colloid in milligrams (mg) that must be added to 10 mL of a standard red gold sol to prevent its coagulation when 1 mL of a 10% sodium chloride (NaCl) solution is added to it.

2. Relationship between Gold Number and Protective Action:
A smaller gold number means that a very tiny amount of the lyophilic colloid is sufficient to prevent the coagulation of the gold sol. Therefore, the protective action or efficiency of a colloid is inversely proportional to its gold number:
Protective Power ∝ 1 / Gold Number

3. Comparing the Given Lyophilic Colloids:
Let us write down the gold number ranges of the given colloids:
Colloid A: 0.005–0.01
Colloid B: 0.15–0.25
Colloid C: 0.04–1.0
Colloid D: 15–25

Comparing these values, Colloid A has the smallest gold number range (0.005–0.01). Because it has the lowest gold number, it requires the minimum quantity to protect the sol from coagulation, making it the most efficient or better protective colloid among the choices provided.