Explanation
FlexVPN supports IKEv2 -> Answer A is not correct.
DMVPN supports both IKEv1 & IKEv2 -> Answer B is not correct.
FlexVPN support multiple SAs -> Answer D is not correct.

Explanation

Explanation
Stateful failover for IP Security (IPsec) enables a router to continue processing and forwarding IPsec packets after a planned or unplanned outage occurs. Customers employ a backup (secondary) router that automatically takes over the tasks of the active (primary) router if the active router loses connectivity for any reason. This failover process is transparent to users and does not require adjustment or reconfiguration of any remote peer.
Stateful failover for IPsec requires that your network contains two identical routers that are available to be either the primary or secondary device. Both routers should be the same type of device, have the same CPU and memory, and have either no encryption accelerator or identical encryption accelerators.
Prerequisites for Stateful Failover for IPsec
Complete, Duplicate IPsec and IKE Configuration on the Active and Standby Devices This document assumes that you have a complete IKE and IPsec configuration. The IKE and IPsec configuration that is set up on the active device must be duplicated on the standby device. That is, the crypto configuration must be identical with respect to Internet Security Association and Key Management Protocol (ISAKMP) policy, ISAKMP keys (preshared), IPsec profiles, IPsec transform sets, all crypto map sets that are used for stateful failover, all access control lists (ACLs) that are used in match address statements on crypto map sets, all AAA configurations used for crypto, client configuration groups, IP local pools used for crypto, and ISAKMP profiles. Reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/sec_conn_vpnav/configuration/15-mt/sec-vpnavailability-15-mt-book/sec-state-fail-ipsec.html Although the prerequisites only stated that "Both routers should be the same type of device" but in the "Restrictions for Stateful Failover for IPsec" section of the link above, it requires "Both the active and standby devices must run the identical version of the Cisco IOS software" so answer E is better than answer B.
This document assumes that you have a complete IKE and IPsec configuration.
The IKE and IPsec configuration that is set up on the active device must be duplicated on the standby device.
That is, the crypto configuration must be identical with respect to Internet Security Association and Key Management Protocol (ISAKMP) policy, ISAKMP keys (preshared), IPsec profiles, IPsec transform sets, all crypto map sets that are used for stateful failover, all access control lists (ACLs) that are used in match address statements on crypto map sets, all AAA configurations used for crypto, client configuration groups, IP local pools used for crypto, and ISAKMP profiles.
Reference:
Although the prerequisites only stated that "Both routers should be the same type of device" but in the Complete, Duplicate IPsec and IKE Configuration on the Active and Standby Devices This document assumes that you have a complete IKE and IPsec configuration. The IKE and IPsec configuration that is set up on the active device must be duplicated on the standby device. That is, the crypto configuration must be identical with respect to Internet Security Association and Key Management Protocol (ISAKMP) policy, ISAKMP keys (preshared), IPsec profiles, IPsec transform sets, all crypto map sets that are used for stateful failover, all access control lists (ACLs) that are used in match address statements on crypto map sets, all AAA configurations used for crypto, client configuration groups, IP local pools used for crypto, and ISAKMP profiles. Reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/sec_conn_vpnav/configuration/15-mt/sec-vpnavailability-15-mt-book/sec-state-fail-ipsec.html Although the prerequisites only stated that "Both routers should be the same type of device" but in the "Restrictions for Stateful Failover for IPsec" section of the link above, it requires "Both the active and standby devices must run the identical version of the Cisco IOS software" so answer E is better than answer B.