A company's employees were victims of a spear phishing campaign impersonating the CEO. The company would now like to implement a solution to improve the overall security posture by assuring their employees that email originated from the CEO. Which of the following controls could they implement to BEST meet this goal?

A$.$

Role based access control $($RBAC$)$

B$.$

IPS

C$.$

Digital Certificates

D$.$

Digital Signatures