DHCP does not work on subnets attached to a router that don't have a DHCP server on them. What is the most probable cause?

DHCP does not work on subnets attached to a router that don't have a DHCP server on them. What is the most probable cause?

• No DHCP relay is set on the router interfaces
• The DHCP server is down
• You need a DHCP server for every subnet and vlan
• This is not possible 

EXPLANATION

Dynamic Host Configuration Protocol (DHCP) servers provide configuration parameters to DHCP clients. DHCP supplies network settings, including the host IP address, the default gateway, and a DNS server. When DHCP clients and associated servers do not reside on the same IP network or subnet, a DHCP relay agent can transfer DHCP messages between them. The DHCP relay agent operates as the interface between DHCP clients and the server. It listens for client requests and adds vital configuration data, such as the client's link information, which is needed by the server to allocate the address for the client. When the DHCP server responds, the DHCP relay agent forwards the reply back to the DHCP client.  

SOURCE

 https://www.cisco.com/c/en/us/td/docs/app_ntwk_services/data_center_app_services/ace_appliances/vA4_1_0/configuration/rtg_brdg/guide/rtbrgdgd/dhcp.pdf