To answer this question, the user needs to understand the different types of access control mechanisms that can be used to provide access to an SSO application in a portal.

A. Mandatory access control: This access control mechanism is typically used in secure environments such as military or government settings. It is a strict access control mechanism that assigns access levels to users based on their security clearance level. This approach is not suitable for providing access to SSO applications in a portal.

B. Role Based Access Control: This access control mechanism assigns roles to users based on their job functions and responsibilities. The roles are used to determine what type of access a user has to an SSO application. This approach is suitable for providing access to SSO applications in a portal.

C. Discretionary Access Control: This access control mechanism allows users to determine who has access to their resources. This approach is not suitable for providing access to SSO applications in a portal.

D. Biometric access control: This access control mechanism uses biometric data such as fingerprints or facial recognition to authenticate users. This approach is not suitable for providing access to SSO applications in a portal.

Therefore, the best approach to be used while providing access to SSO application in a portal is Role Based Access Control.

The Answer is: B