You need to ensure that users can authenticate to Office 365

An organization implements single sign-on (SSO) for use with Office 365 services. You install an
Active Directory Federation Services (AD FS) proxy server.
Users report that they are unable to authenticate. You launch the Event Viewer and view the event
information shown in the following screen shot:

You need to ensure that users can authenticate to Office 365.
What should you do?

A.
Re-enter the credentials used to establish the trust.

B.
Verify the federation server proxy is trusted by the federation service.

C.
Re-install the Secure Sockets Layer (SSL) certificate for the federation service.

D.
Verify network connectivity between the Federation Service Proxy and federation server.