Explanation

Explanation
In the 5GHz spectrum some of the channels used by 802.11 are subject to Dynamic Frequency Selection (DFS) requirements. This is due to our clients coexistence with other RF technologies such as Maritime, Aviation and Weather RADAR.
Dynamic Frequency Selection (DFS) is the process of detecting radar signals that must be protected against interference from 5.0 GHz (802.11a/h) radios, and upon detection switching the operating frequency of the 5.0 GHz (802.11a/h) radio to one that is not interfering with the radar systems.
Reference: https://www.cisco.com/en/US/docs/routers/access/wireless/software/guide/RadioChannelDFS.pdf Although DFS helps reduce interference with radar systems but "DFS channels" refer to the 5GHz channels that require DFS check. In other words, DFS channels are channels that may interfere with radar signal.
Therefore we should disable these DFS channels -> Answer C is correct.
UNII-2 (5.250-5.350 GHz and 5.470-5.725 GHz) which contains channels 52, 56, 60, 64, 100, 104, 108, 112,
116, 120, 124, 128, 132, 136, and 140 are permitted in the United States, but shared with radar systems.
Therefore, APs operating on UNII-2 channels are required to use Dynamic Frequency Selection (DFS) to avoid interfering with radar signals. If an AP detects a radar signal, it must immediately stop using that channel and randomly pick a new channel.
Reference:
https://documentation.meraki.com/MR/WiFi_Basics_and_Best_Practices/Channel_Planning_Best_Practices

Explanation

Explanation
When OSPF adjacency is formed, a router goes through several state changes before it becomes fully adjacent with its neighbor. The states are Down -> Attempt (optional) -> Init -> 2-Way -> Exstart -> Exchange -> Loading -> Full. Short descriptions about these states are listed below:
Down: no information (hellos) has been received from this neighbor.
Attempt: only valid for manually configured neighbors in an NBMA environment. In Attempt state, the router sends unicast hello packets every poll interval to the neighbor, from which hellos have not been received within the dead interval.
Init: specifies that the router has received a hello packet from its neighbor, but the receiving router's ID was not included in the hello packet
2-Way: indicates bi-directional communication has been established between two routers.
Exstart: Once the DR and BDR are elected, the actual process of exchanging link state information can start between the routers and their DR and BDR.
Exchange: OSPF routers exchange database descriptor (DBD) packets
Loading: In this state, the actual exchange of link state information occurs Full: routers are fully adjacent with each other (Reference: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f0e.s html) Neighbors Stuck in Exstart/Exchange State the problem occurs most frequently when attempting to run OSPF between a Cisco router and another vendor's router. The problem occurs when the maximum transmission unit (MTU) settings for neighboring router interfaces don't match. If the router with the higher MTU sends a packet larger that the MTU set on the neighboring router, the neighboring router ignores the packet.