What should an administrator enforce to prevent any public IP assignments on network interfaces?

An Azure Policy Definition is the correct choice because it allows administrators to create, assign, and manage policies that enforce rules and effects over your resources. By defining a policy that disallows public IP assignments on network interfaces, Azure will prevent the creation or modification of resources that violate this rule. Azure Sentinel is a SIEM service for security analytics, not for enforcing resource configuration. Azure Resource Graph is a service for exploring and querying resources, not for enforcing policies. Azure Role Assignments control who can do what with resources, but they don't enforce configuration rules like preventing public IP assignments.