When mercuric iodide is added to the aqueous solution of potassium iodide, the

When mercuric iodide is added to aq. sol. of $Kl$ the freezing point of the mentioined solution increases. Mercuric Iodide will react with Potassium Iodide to form dipotassium tetraiodomercury
This will decrease the number of ions in the solution and so Van't Hoff factor decreases which depend on the number of particles.

Thus, Option A is the correct answer