Which approach ensures that an attacker WIFI need to crack the passwords one at a time?

A web application is being developed to provide online sales to a retailer. The customers will need to use their username and passwords to login into then profile and complete their order For this reason the application must store user passwords.

Which approach ensures that an attacker WIFI need to crack the passwords one at a time?
A . Apply the peppering technique
B . Store the passwords by using asymmetric encryption
C . Apply the salting technique
D . Store the passwords by using symmetric encryption

Answer: C