Gold number of protective colloids A, B, C and D are 0.50, 0.01, 0.05 and0.005 respectively. The correct order of their protective power is

A < C < B < D

The correct answer is option 1.  A < C < B < D.

The protective power of a protective colloid is measured by its gold number, which indicates the minimum amount of the colloid required to prevent the coagulation of a gold sol under standardized conditions.

The lower the gold number, the higher the protective power of the colloid. This is because a smaller amount of the colloid is needed to effectively prevent the coagulation of the dispersed particles (in this case, the gold sol).

In the given scenario:

Colloid D has the lowest gold number (0.005), indicating the highest protective power.

Colloid B has the second lowest gold number (0.01), indicating the second highest protective power.

Colloid C has the third lowest gold number (0.05), indicating the third highest protective power.

Colloid A has the highest gold number (0.50), indicating the lowest protective power.

So, the correct order of their protective power is: A < C < B < D.

This means that colloids with lower gold numbers are more effective in preventing the coagulation of the dispersed particles, while those with higher gold numbers are less effective and require larger amounts to achieve the same level of protection.