Users report issues with reachability between areas as soon as an engineer configured summary routes...

For OSPF, we can only summary at the ABR with the command “area range” or at the ASBR with the command “summary-address” - > Therefore answer A and answer B are not correct.

In this question, the most likely problem is that when doing summarization, the network mask is configured wrong and summarization doesn’t work because of the misconfiguration. When configuring the area range command, make sure that the summarization mask is in the form of a prefix mask rather than a wildcard mask (that is, 255.255.255.0 instead of 0.0.0.255).

Good reference: https://www.configrouter.com/troubleshooting-route-summarization-ospf-14082/