How will the default route configured on R1 affect the operation of R2?

First, the more-specific routes will always be favored over less-specific routes regardless of the administrative distance set for a protocol.

In this case, because we use OSPF for three networks
(172.16.100.0 0.0.0.3, 172.16.100.64 0.0.0.63, 172.16.100.128 0.0.0.31) so the packets destined for these networks will not be affected by the default route.

The default route configured on R1 “ip route 0.0.0.0 0.0.0.0 serial0/0” will send any packet whose destination network is not referenced in the routing table of router R1 to R2, it doesn’t drop anything so answers A, B and C are not correct.

D is not correct too – the routes are declared in R1 and the question says that “OSPF has been correctly configured on router R2”, so network directly connected to router R2 can communicate with those three subnetworks.

So the default route configured on R1 will send any packet destined for a network that is not referenced in its routing table to R2

R2 in turn sends it to R1 because it is the only way and a routing loop will occur.