There isn't another CT, only taps to adjust the way the two currents oppose each other in the restraint coils. Think about it this way: the operating coil shouldn't see much current under full load, so the taps selected need to balance the currents through the restraint coils. With one current at 4.8 A and its tap set at 5 A and the other at 8.3 A at full load, the other tap should be set slightly above 8.3 A so the op coil sees no current. Hence, 8.5:5 is the best answer.
Honestly, I hate this question, since:
- it applies to electromechanical relays, which you don't see much of any more, and the ones that are in service already have their taps set
- it uses delta-connected CTs on the wye side of the transformer, which isn't normally done any longer, as with modern digital relays you simply use wye-connected CTs and tell them the transformer connections and they do the math