What is the molarity when water is added to 4 moles of sodium chloride to make 0.5 liter of solution?

       If    0.5 L of solution contains 4 mol
then let     1 L of solution contain  x  mol
      
       ⇒    (0.5 L) x  = (4 mol) (1 L)

                         x  = (4 mol · L)  ÷  (0.5 L)

                         x  = 8 mol

Thus the molarity of the Sodium Chloride solution is 8 mol / L   OR  8 mol/dm³. 

The answer is probably c. xylem