Very subjective, very difficult to answer. I have not found people good at NW so I would normally not ask technical questions but only general questions.

I suggest you have a look at the NW admin guide.

Have a general knowledge on most of the chapters.
Backups
Restore
Configurations / client / server / storage Node / Groups / schedules
DISASTER RECOVERY
NW Platform ( The NW zone)
Connectivity - LAN / WAN
Save Sets
Scheduled Backup / CAHDOC backup.
Possible problems and how to go about sorting out problems..
Compatibility with OS / and Hardware requirements.
Licensing
Volume rotations for a proposed backup solutions..

and so on and so forth.....

What I know from many people is that they ask more about DR of NW server, recovering indexes or RES folder, command line utilities etc.

Personally, I would not recommended product specific jobs, they restrict your technical growth. I hope ITC gives you enough exposure on multiple things then restricting to one thing.

I would not ask questions... I would give them a system with problem and ask to fix it by telling me exactly what s(he) does and thinks. This way I would see how good that person might be and what experience does it have.