A&C...The quorum rule states that to make changes to the configuration in our cluster, we need to have more than half of the nodes available. This prevents configuration conflicts if we lose connectivity between the nodes in our cluster. So, if we have only 2 racks and 4 nodes we could never get more than half of the nodes available if one of those racks power fails. We need 6 nodes separated on 3 racks, so we can have more than half of the nodes available if one of those racks power fails.

B & D are correct. Because there is the possibility of a tie in a cluster that has an even number of nodes, one node has an extra fractional voting weight called epsilon. If the connectivity between two equal portions of a large cluster fails, the group of nodes containing epsilon maintains quorum, assuming that all of the nodes are healthy. For example, the following illustration shows a four-node cluster in which two of the nodes have failed. However, because one of the surviving nodes holds epsilon, the cluster remains in quorum even though there is not a simple majority of healthy nodes. https://docs.netapp.com/us-en/active-iq-unified-manager/health-checker/concept_understand_quorum_and_epsilon.html