< b="" />Explanation:< />

Ferric hydroxide (Fe(OH)3) is a negative sol. A negative sol is a colloidal solution in which the dispersed phase has a negative charge. In this case, the Fe(OH)3 particles have a negative charge.

To coagulate a negative sol, we need to add an electrolyte that can neutralize the negative charges on the particles, causing them to come closer together and eventually form a precipitate or coagulated mass.

Let's analyze the options given:

a) FeCl3:
FeCl3 is an electrolyte that dissociates into Fe3+ and Cl- ions in solution. The Fe3+ ions can neutralize the negative charges on the Fe(OH)3 particles, leading to their coagulation.

b) CaCO3:
CaCO3 is not an electrolyte. It is a sparingly soluble salt that does not dissociate into ions in solution. Therefore, it cannot neutralize the negative charges on the Fe(OH)3 particles effectively and will not cause coagulation.

c) BaSO4:
BaSO4 is also not an electrolyte. It is a highly insoluble compound that does not dissociate into ions in solution. Like CaCO3, it cannot effectively neutralize the negative charges on the Fe(OH)3 particles and will not cause coagulation.

d) NaCl:
NaCl is a common electrolyte that dissociates into Na+ and Cl- ions in solution. However, the Cl- ions have a negative charge, which will not effectively neutralize the negative charges on the Fe(OH)3 particles. Therefore, NaCl is not the best option for coagulating Fe(OH)3.

Therefore, the correct answer is option 'A' - FeCl3. FeCl3 is an electrolyte that can effectively neutralize the negative charges on the Fe(OH)3 particles, leading to their coagulation.