A 1 liter aqueous solution contains 0.34 M ammonia. Of the options provided to convert this solution into a buffer by adding various substances while keeping the volume constant, the best option is to add 0.35 mol of NH4NO3. NH4NO3 is suitable because when added to the solution containing NH3, it would create a basic buffer system of the weak base NH3 and its salt with the strong acid HNO3.