Explanation
The network 20.0.0.0 0.0.0.255 area 0 command on R2 did not cover the IP address of Fa1/1 interface of R2 so OSPF did not run on this interface. Therefore we have to use the command network 20.1.1.2 0.0.255.255 area 0 to turn on OSPF on this interface.
Note: The command network 20.1.1.2 0.0.255.255 area 0 can be used too so this answer is also correct but answer C is the best answer here.
The network 0.0.0.0 255.255.255.255 area 0 command on R1 will run OSPF on all active

Explanation
There are five basic device roles in the fabric overlay:
+ Control plane node: This node contains the settings, protocols, and mapping tables to provide the endpoint-to-location (EID-to-RLOC) mapping system for the fabric overlay.
+ Fabric border node: This fabric device (for example, core layer device) connects external Layer 3 networks to the SDA fabric.
+ Fabric edge node: This fabric device (for example, access or distribution layer device) connects wired endpoints to the SDA fabric.
+ Fabric WLAN controller (WLC): This fabric device connects APs and wireless endpoints to the SDA fabric.
+ Intermediate nodes: These are intermediate routers or extended switches that do not provide any sort of SD-Access fabric role other than underlay services.