You have a Windows Server 2008 R2 that has the Active Directory Certificate Services
server role installed.
You need to minimize the amount of time it takes for client computers to download a
certificate revocation list (CRL).
What should you do?

A.
Install and configure an Online Responder.

B.
Import the Issuing CA certificate into the Trusted Root Certification Authorities store on all
client workstations.

C.
Install and configure an additional domain controller.

D.
Import the Root CA certificate into the Trusted Root Certification Authorities store on all
client workstations.

Explanation:
http://technet.microsoft.com/en-us/library/cc725958.aspx
What Is an Online Responder?
An Online Responder is a trusted server that receives and responds to individual client
requests for information about the status of a certificate.
The use of Online Responders is one of two common methods for conveying information
about the validity of certificates. Unlike certificate revocation lists (CRLs), which are
distributed periodically and contain information about all certificates that have been revoked
or suspended, an Online Responder receives and responds only to individual requests from
clients for information about the status of a certificate. The amount of data retrieved per
request remains constant no matter how many revoked certificates there might be.
In many circumstances, Online Responders can process certificate status requests more
efficiently than by using CRLs.