Final answer:
Password expiration policies are the best method to ensure regular updating of passwords for enhanced security. It should be combined with other practices like two-factor authentication and password complexity for optimal security.
Step-by-step explanation:
The best method to promote a secure environment by requiring users to change their passwords after a certain period is D) Password expiration policies. This method ensures that passwords are updated regularly, which helps in reducing the risk of unauthorized access as older passwords are phased out. However, for optimal security, it's also important to incorporate other practices such as two-factor authentication, enforcing password complexity requirements, and providing employee education to avoid phishing and other scams.