The command "crypto isakmp identity address 172.19.20.24" is not valid. We can only use "crypto isakmp identity {address | hostname}. The following example uses preshared keys at two peers and sets both their ISAKMP identities to the IP address.
At the local peer (at 10.0.0.1) the ISAKMP identity is set and the preshared key is specified:
crypto isakmp identity address
crypto isakmp key sharedkeystring address 192.168.1.33
At the remote peer (at 192.168.1.33) the ISAKMP identity is set and the same preshared key is specified:
crypto isakmp identity address
crypto isakmp key sharedkeystring address 10.0.0.1
Reference:
The command "crypto enrollment peer address" is not valid either.
The command "crypto ca identity ..." is only used to declare a trusted CA for the router and puts you in the caidentity configuration mode. Also it should be followed by a name, not an IP address. For example: "crypto ca identity CA-Server" -> Answer A is not correct.
Only answer B is the best choice left.