I don't know the answer either.
Ground...because the others control you in the air?
Radar...because the others are used for arriving/departing traffic, radar only if you're passing by (er...I think?)
It's a really really unfair question!!!
Seriously, if I got that question, and it really mattered (eg, I failed by a couple of marks) I'd take it up with the CAA, who are not unreasonable about such things if you argue your case well.