i-rule on an LTM for returning a DNS response based on the zone value in the query

You are quite restricted in which DNS events and commands you can use without a GTM or DNS Services add-on license. I believe the only event you can trigger an iRule on is DNS_RESPONSE. You have access to the DNS::answer and DNS::type commands, so I suppose you could parse the answer on the DNS_RESPONSE event to see if the response includes an A record in zone in abc.net. If so, you can replace DNS::answer with the answer of your choice. See https://clouddocs.f5.com/api/irules/DNS.html for more details.