Final answer:
The most common form of authentication is passwords because they are easy to implement and use. Other forms such as PINs, digital certificates, and smart cards are also used, but less frequently.
Step-by-step explanation:
The most common form of authentication is password authentication. This method requires users to provide a secret word or phrase that only they should know. Passwords are widely used across various platforms and services because they are easy to implement and use. However, they must be complex enough to resist brute-force attacks and should not be reused across different sites for security reasons.
While PINs (Personal Identification Numbers), digital certificates, and smart cards are other forms of authentication, they are not as commonly used as passwords. PINs are typically associated with banking and secure transactions, digital certificates are used for encrypted communication, and smart cards are used for physical and logical access control in secure facilities.