Your network contains an Active Directory domain named adatum.com. All servers run Windows Server 2012
R2. All domain controllers have the DNS Server server role installed.You have a domain controller named DC1.
On DC1, you create an Active Directory-integrated zone named adatum.com and you sign the zone by using
DNSSEC.
You deploy a new read-only domain controller (RODC) named R0DC1.
You need to ensure that the contoso.com zone replicates to RODC1.
What should you configure on DC1?
To answer, select the appropriate tab in the answer area.
Point and Shoot:
Answer: 
Explanation:
http://technet.microsoft.com/en-us/library/cc781340(v=ws.10).aspx
For additional servers to host a zone, zone transfers are required to replicate and synchronize all copies of the
zone used at each server configured to host the zone.
I think the given explanation points to outdated and not appropriate content (retired Server 2003 docs).
This one fits better:
https://technet.microsoft.com/en-us/library/dn593674(v=ws.11).aspx#rodc
“In this scenario, you must manually enable zone transfers on the primary server that is selected by the RODC”
Thanks Den, your link does explain it better
Read-only domain controllers (RODCs)
In Windows Server 2008 and Windows Server 2008 R2, DNS servers running on read-only domain controllers (RODCs) host Active Directory-integrated copies of all zones. However, because the zone is read-only, the DNS server cannot make any updates to the zones that it hosts. Instead, updates occur on other DNS servers and are transferred to the RODC through Active Directory replication.
When an Active Directory-integrated zone is signed with DNSSEC, private keys are also replicated to all DNS servers running on domain controllers, with an exception: Private keys are not replicated to an RODC because RODCs are intended to operate in insecure environments.
In Windows Server 2012 and Windows Server 2012 R2, an RODC loads unsigned zones from Active Directory with no change in functionality from Windows Server 2008 R2. However, if the RODC finds a DNSSEC-signed zone in Active Directory, it does not load the zone as Active Directory-integrated. Instead, it creates a secondary copy of the zone, and then configures the closest writeable domain controller for the domain as the primary server. The RODC then attempts to perform a zone transfer. Zone transfers must be enabled on the primary DNS server for this transfer to succeed. If zone transfers are not enabled, the RODC logs an error event and takes no further action. In this scenario, you must manually enable zone transfers on the primary server that is selected by the RODC. Alternately, you can choose to reconfigure the RODC to point to a different primary DNS server that has zone transfers enabled.