We are asked to find the proton concentration of a 5 M solution of HCl in water. Since HCl is a strong acid, we know that it completely ionizes in water. That means that it is separated into its respective ions in solution. The ionization of HCl is shown below.
HCl --> H+ + Cl-
Since it completely ionizes, that means that all of the HCl is converted to H+ and Cl-. So a 5 M solution of HCl will give a solution that is 5 M in H+ and 5 M in Cl-. So the answer is 5 M concentration of protons.