Answering this type of question requires you to know the relationships between stall speed, weight and load factor.
These relationships include:
1. Stall speed in straight and level flight is proportional to the square root of the weight.
This gives us the equation Vs at new weight = Vs at old weight x square root of( new weight / old weight ).
2. Stall speed in any manoeuvre is proportional to the square root of the load factor.
This gives us the equation Vs in any manoeuvre = Vs is Straight and level flight x the square root of the load factor in the manoeuvre.
In this question we have load factor increasing from 1 in straight and level flight to 1.15 in the turn.
So the new stall speed is proportional to the square root of 1.15, which is approximately 1.07. This means an increase of 7%.