\Coagulating power of an ion for a colloidal solution depends on:

The correct option is: Its size and magnitude of charge both.

To understand why the coagulating power of an ion for a colloidal solution depends on both its size and the magnitude of its charge, we can look at the physical chemistry principles of colloidal stability.

Colloidal particles carry a charge on their surface, which keeps them dispersed in the medium due to electrostatic repulsion. To coagulate or precipitate these particles, an electrolyte must be added. The ions of the electrolyte carrying a charge opposite to that of the colloidal particles (called coagulating ions or counter-ions) neutralize the charge on the colloid, causing coagulation.

According to the Hardy-Schulze Rule, the coagulating power of an ion is directly proportional to the magnitude of its charge. The greater the valence (or charge) of the active ion, the greater is its power to cause coagulation. For example, for a negatively charged arsenious sulfide sol, the coagulating power of cations follows the order:
${\mathrm{Al}}^{3+}>{\mathrm{Ba}}^{2+}>{\mathrm{Na}}^{+}$

In addition to the magnitude of the charge, the size of the ion also plays a significant role. Smaller ions have a higher charge density (charge-to-size ratio) compared to larger ions of the same charge. Ions with higher charge density are more effective at polarizing and neutralizing the charges on the colloidal particles because they can approach the surface of the colloidal particles closely. Thus, both the size and the magnitude of the charge of the ion determine its overall coagulating power.