The coagulation of a sol occurs when the dispersed particles come together and form larger aggregates or clusters. This process can be achieved by adding an electrolyte solution to the sol, which causes the charged particles in the sol to neutralize and come closer together.

To determine the most effective electrolyte solution for coagulating the arsenic sulphide sol, we need to consider the charge and valency of the ions present in each solution. The higher the charge and valency of the ions, the greater the ability to neutralize the charged particles in the sol and cause coagulation.

Let's analyze each option:

a) KCl:
- K+ ions have a charge of +1
- Cl- ions have a charge of -1

b) MgCl2:
- Mg2+ ions have a charge of +2
- Cl- ions have a charge of -1

c) AlCl3:
- Al3+ ions have a charge of +3
- Cl- ions have a charge of -1

d) Na3PO4:
- Na+ ions have a charge of +1
- PO43- ions have a charge of -3

From the analysis above, we can see that the AlCl3 solution has the highest charge and valency of ions. The Al3+ ions in the solution have a charge of +3, which is greater than the charges of the ions in the other solutions. This high charge allows the Al3+ ions to effectively neutralize the charged particles in the arsenic sulphide sol, causing coagulation.

On the other hand, the other solutions have lower charges and valencies of ions, which means they may not be as effective in neutralizing the charged particles in the sol. Therefore, option 'C' (AlCl3) is the most effective electrolyte solution for causing the coagulation of the arsenic sulphide sol.

In conclusion, the AlCl3 solution will be the most effective in causing the coagulation of the arsenic sulphide sol due to its high charge and valency of ions.