The strongest security possible for SNMPv3 requires both authentication and encryption, which is achieved by using the priv security level. Authentication ensures that the message is from a valid source, and encryption scrambles the content of the packet to prevent it from being learned by an unauthorized source.
The auth sha and priv aes 256 parameters specify the algorithms used for authentication and encryption, respectively. SHA is more secure than MD5, and AES 256 is more secure than DES or 3DES. Therefore, option D is the correct answer, as it uses the priv security level, the SHA algorithm for authentication, and the AES 256 algorithm for encryption. The other options either use a lower security level (noauth or authNoPriv), a weaker encryption algorithm (des or 3des), or no encryption at all. References :=
* SNMP Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3850 Switches) - SNMP Version 3
* Configuration Template for SNMPv3 - Cisco Community
* SNMP Version 3 - Cisco