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.

Explanation
When supporting personal devices on a corporate network, you must protect network services and enterprise data by authenticating and authorizing users (employees, contractors, and guests) and their devices. Cisco ISE provides the tools you need to allow employees to securely use personal devices on a corporate network.
Guests can add their personal devices to the network by running the native supplicant provisioning (Network Setup Assistant), or by adding their devices to the My Devices portal.
Because native supplicant profiles are not available for all devices, users can use the My Devices portal to add these devices manually; or you can configure Bring Your Own Device (BYOD) rules to register these devices.
Reference:
/m_ise_devices_byod.html

Cisco IOS public key infrastructure (PKI) provides certificate management to support security protocols such as IP Security (IPSec), secure shell (SSH), and secure socket layer (SSL). This module identifies and describes concepts that are needed to understand, plan for, and implement a PKI.
A PKI is composed of the following entities: ...
- A distribution mechanism (such as Lightweight Directory Access Protocol [LDAP] or HTTP) for certificate revocation lists (CRLs)

Explanation/Reference: https://www.cisco.com/c/en/us/products/security/pxgrid.html

Explanation