What mechanism should an Architect put in place to enable a trusted connection between the login service and Salesforce?

Universal Containers (UC) is setting up delegated authentication to allow employees to log in using their corporate credentials. UC’s security team is concerned about the risks of exposing the corporate login service on the internet and has asked that a reliable trust mechanism be put in place between the login service and Salesforce.

What mechanism should an Architect put in place to enable a trusted connection between the login service and Salesforce?
A . Require the use of Salesforce security tokens on passwords.
B . Enforce mutual authentication between systems using SS
D . Include Client Id and Client Secret in the login header callout.
E . Set up a proxy service for the login service in the DM

Answer: A