Question

The coagulating power of electrolytes having ions Na$^+$, Al$^{3+}$ and Ba$^{2+}$ for As$_2$S$_3$ sol increases in the order

• A. Al$^{3+}$ $<$ Ba$^{2+}$ $<$ Na$^{+}$
• B. Na$^{+}$ $<$ Ba$^{2+}$ $<$ Al$^{3+}$
• C. Ba$^{2+}$ $<$ Na$^{+}$ $<$ Al$^{3+}$
• D. Al$^{3+}$ $<$ Na$^{+}$ $<$ Ba$^{2+}$

Hint:

Using a simple frame or just bolding for the box
Key Points: 
As$_2$S$_3$ sol is negatively charged. 
Coagulation of negatively charged sols is caused by cations. 
Hardy-Schulze Rule: Coagulating power increases significantly with increasing charge magnitude of the coagulating ion. 
Cation Charges: Na$^{+}$ (+1), Ba$^{2+}$ (+2), Al$^{3+}$ (+3). 
Increasing Coagulating Power: Na$^{+}$ $<$ Ba$^{2+}$ $<$ Al$^{3+}$.