How many servers should you deploy?

You are the Office 365 administrator for your company. The company has a single office.
You have the following requirements:
 You must configure a redundant Active Directory Federation Services (AD FS) implementation.
 You must use a Windows Internal Database to store AD FS configuration data. The solution must use a custom login page for external users.
 The solution must use single sign-on for internal users.
You need to deploy the minimum number of servers.
How many servers should you deploy?

A.
2

B.
4

C.
6

D.
16