FLG practice question help
Guys, I am getting really stumped on this question, and reviewing Cisco docs on CEF adjacencies is not helping 
Here is the question..
The correct answer is E, but i'm having trouble figuring out why and also proving the other answers to be false. A and D seem to be easier to rule out, because the prefix has to exist in the routing table for CEF to know about it, and typically disabling and re-enabling an IOS feature isn't the "Cisco" preferred way of troubleshooting for exams although I've seen it mentioned as a workaround in IOS caveats more than once for CEF.
Here is the question..
Which of the following is the solution if part of an adjacency is an "incomplete" or a "drop" adjacency?
a. Troubleshoot routing protocols to ensure that the routing table is properly populated
b. Verify that the IP host exists on a media that is accessible to the host that is trying to ARP resolve.
c. Debug CEF to observe all messages for CEF operations.
d. Disable and then re-enable CEF on the multilayer switch.
e. Check that unsupported software features have not been enabled.
The correct answer is E, but i'm having trouble figuring out why and also proving the other answers to be false. A and D seem to be easier to rule out, because the prefix has to exist in the routing table for CEF to know about it, and typically disabling and re-enabling an IOS feature isn't the "Cisco" preferred way of troubleshooting for exams although I've seen it mentioned as a workaround in IOS caveats more than once for CEF.
Cisco was my first networking love, but my "other" router is a Mikrotik...