What is a required attribute to configure NTP authentication on a Cisco ASA?

Answer is correct:

“The Authentication Key is available but not necessary. The time server will have a key, and you will create a key number and associate a key value. The same key, once created, can be assigned for use with multiple servers. This makes it somewhat easier to set up authenticated NTP on an internal network, as you can set up the routers on your internal network to serve time and require the same key.”

https://docs.secureauth.com/display/docs/Configuring+NTP+on+the+Cisco+ASA