Which port should to open on the firewall?

You have two servers named Server1 and Server2.
A firewall exists between Server1 and Server2.
Both servers run Windows Server Update Services (WSUS).
Server1 downloads updates from Microsoft update.
Server2 must synchronize updates from Server1.
Which port should to open on the firewall?
A. 80
B. 443
C. 3389
D. 8530