A security engineer needs to implement password authentication on a highly specialized system. A requirement is that if two different users specify the same password, the stored digests will not be the same. How can this be accomplished?

a$.$ Use a different asymmetrical encryption key for each user.

b$.$ Use SHA instead of MD$5$ as the hashing algorithm to create the digest.

c$.$ Implement salting to make dictionary and brute$-$force attacks more difficult.

d$.$ Use peppering techniques that slightly modify the password hashing function.