Which authentication approach should you recommend?

You are designing a SharePoint 2010 application that contains a Web Part. The Web Part must connect to an
RSS data stream provided by a Web service. The Web service is on another server in the domain. You need to
recommend an authentication approach that meets the following requirements:
SharePoint users must not be required to supply additional credentials for the Web service.
.Standard authentication systems available to SharePoint 2010 must be used.
Which authentication approach should you recommend?

A.
NTLM

B.
Forms-based authentication (FBA)

C.
Kerberos

D.
Lightweight Directory Access Protocol (LDAP)