To prepare a 2 mol·L⁻¹ solution of KNO₃, you must measure the mass of the A. solute and the volume of the B. solution. 
The formula for molar concentration (c) is 
c = moles of solute/litres of solution 
∴ You measure out a mass of KNO₃ equivalent to two moles (202.20 g), and then you add enough water to make the volume of solution up to 1 L.