I took it to mean whilst banking at 30˚ you need an EXTRA 15 lift which is what stumped me and I still wouldn't know how to solve that.
Well to maintain altitude during a 30 degree banked turn you need to increase lift by 15%. This gives 1.15 times the straight and level lift.
If we now interpret the question as meaning that we need an additional 15% on top of this, we will have 1.15 x 1.15 = 1.3225 times the straight and level lift. Or 1.15 + 0.15 = 1.3 if you take the "additional 15%" as meaning 15% of the S+L lift.
If we now use the equation
Vs in a manoeuvre = Vs in straight and level x square root of load factor we have
Vs = 1 x square root of 1.3225 = 1.15. (Or 1 x square root of 1.3 = 1.14.)
This means that our stall speed has increased by 15%. (Or 14%, which is not an option)
No matter how much effort the examiners put into constructing their questions, it will always be possible for some readers to interpret them in unexpected ways. Personally I would interpret this question as being about a simple constant altitude 15 degree banked turn, in which case the answer is 7%.
It is worth noting that we can carry out a banked turn without any increase in lift, but this will cause the aircraft to sink. If this is a real JAR/EASA ATPL exam question, the use of the words "additional 15% lift" could be the examiner's way of indicating that he/she meant a constant altitude turn.