To answer the question you need to know two things:
(1) Stall speed increases by the square root of the load factor
(2) In a turn, load factor can be calculated using the formula:
load factor = 1/(cos angle of bank)
Firstly we work out what load factor we would need to stall at 1.4 Vs: 1.4 x 1.4 = 2. Now we know that the load factor required is 2, we can work out the angle of bank that would achieve that load factor. If we rearrange the formula we get: cos angle of bank = 1/load factor; 1/2 = 0.5. Cos-1 0.5 = 60, so (a) is the correct answer.
If you you forget that stall speed increases by the square root of the load factor, rather than the load factor itself, then you get answer (d).
Hope that helps
Al