You have an Exchange organization. All servers in the organization have Exchange Server
2010 Service Pack 1 (SP1) installed.
The organization is authoritative 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.
an A record for autodiscover.nwtraders.com

B.
a CNAME record for nwtraders.com

C.
a TXT record for nwtraders.com

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

Explanation:
See Step 4 Create an Autodiscover DNS record in http://technet.microsoft.com/enus/library/ff601760.aspx
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.

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