Your company has a main office and one branch office. All servers run Windows Server 2003 Service Pack 2 (SP2). The main office has a third-party gateway device named Gateway1. Gateway1 is connected to the internal network and the Internet. Gateway1 supports IPSec. In thebranch office, you have a server named Server1. You create an IPSec policy on Server1. You need to ensure that Server1 can establish an IPSec tunnel to Gateway1. What should you use to configure the IPSec policy?（）

A. an IP filter that allows only Internet Control Message Protocol （ICMP） traffic.
B. an IP filter that allows only TCP traffic on port 1701.
C. a pre-shared key for authentication.
D. Kerberos authentication.