If your wife intends to use your last name as her last name going forward, she should list the new last name in Q.1a. Under Q2a. is where she will need to list her maiden name which is the name on her passport and other documents she currently has. Doing so ensures the GC gets issued in her new last name. Her marriage certificate makes the new last name official. And if she has to travel after getting her GC, and before getting a new passport in her new last name, she just needs to have her marriage certificate on her to show as official document for the change of name.
If she lists her old last name in 1a and the new last name in 2a, the GC will be issued under the old last name. If she subsequently wants to change the last name to reflect her new last name later on, she will need to play for a replacement GC, which is not cheap, and could be waiting for a long time (could be up to one year of more) for the replacement GC to get issued. She will be without a GC during that period as she would have needed to send in the old GC with her application (if I’m not mistaken on this).