Which of the following is the BEST and most efficient solution to deploy?

Recent feedback from an employee engagement survey stated that users are frustrated with multiple logins to different SaaS providers, and the cloud engineering teams is directed to address this issue and implement a solution. The security policy states that users must access the SaaS from approved IP addresses.

Which of the following is the BEST and most efficient solution to deploy?
A . Deploy an IPSec tunnel to each SaaS provider and enable biometric login.
B . Deploy a smart card login and change passwords to each SaaS vendor every 30 days.
C . Deploy a CASB solution and whitelist the approved SaaS applications.
D . Deploy SSO and enforce VPN access to the corporate domain.

Answer: D