Answer: Ask a member of the Enterprise Admins group to authorize the server.
Step-by-step explanation:
From the information given, we are informed that Rachel installed a new server running Windows Server 2016 on the new subnet and configure it as a DHCP server but then realizes that the DHCP server service has started, but that the server does not respond to DHCP lease requests.
In this case, the reason for this can be attributed to the fact that the server hasn't been authorized, therefore, she can ask a member of the Enterprise Admins group to authorize the server.