Answer: 5.0 M NaCl
Step-by-step explanation:
A concentrated solution is one which has more amount of solute in it.
A dilute solution is one which has less amount of solute in it.
Molarity is defined as the number of moles of solute present per liter of the solution.
0.5 M NaCl will have 0.5 moles of NaCl in 1 L of the solution, 0.005 M NaCl has 0.005 moles of NaCl in 1 L of the solution, 0.05 M NaCl has 0.05 moles of NaCl in 1 L and 5.0 M NaCl has 5 moles of NaCl in 1 L.
Thus 5.0 M NaCl will be most concentrated.