The addition of a little of mercuric iodide to the aqueous solution of potassium iodide results into

The addition of a little of mercuric iodide to the aqueous solution of potassium iodide increase in freezing point.

The reduced particle size at the freezing point. It follows that the solution's freezing point will rise. The freezing point is enhanced when mercuric iodide is added to a KI aqueous solution.

Hence, option A is correct.