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