You have an Exchange organization. All servers in the organization have Exchange Server 2010 SP1 installed.

The organization is authorative for a domain named nwtraders.com.

You create a federated trust.

You need to ensure that the federated partner servers can locate the Client Access servers in the organization.

Which record should you create in the external DNS zone?

A.
a TXT record for nwtraders.com

B.
a CNAME record for nwtraders.com

C.
an A record for autodiscover.nwtraders.com

D.
an SRV record for _federation._TCP.nwtraders.com

Explanation:
You need to add an alias canonical name (CNAME) resource record to your public-facing DNS. The new CNAME record should point to an Internet-facing Client Access server that’s running the Autodiscover service.

In the previous Contoso example, the new CNAME record would specify autodiscover.contoso.com as the host name. For organizations using Microsoft DNS, you can add a CNAME record by using either DNS Manager or the DNSCmd command. For detailed instructions, see Add an Alias (CNAME) Resource Record to a Zone.

Reference:

http://technet.microsoft.com/en-us/library/ff601760.aspx (Step 4: Create an Autodiscover DNS record)