The IT administrator for a large university uses a Lightweight Directory Access Protocol $($LDAP$)$ directory service to manage user access to various computing resources. To ensure the directory's security, which of the following measures should the administrator implement?

a$.$

Use the basic LDAP protocol without any additional security mechanisms

b$.$

Use LDAP Secure on port $636$ for secure user credential exchange

c$.$

Use Simple Bind with plaintext of distinguished name and password

d$.$

Allow anonymous access to the directory for easy user onboarding