Final answer:
The default cost for all Active Directory site links is set to 100, which helps determine the replication paths between sites by indicating the relative speed and expense of the replication traffic over the link, the correct option is C).
Step-by-step explanation:
The default value that all site link costs are configured with in Active Directory is 100.
Site links in Active Directory are used to define replication paths between sites, and the cost associated with a site link helps determine the replication route.
The cost is a numerical value that represents the relative expense or speed of replication traffic over that link; a lower cost indicates a preferred path over higher cost paths.
When setting up an Active Directory, it's essential to configure site link costs properly to ensure efficient replication within the network.