The figure shows four steel bars three are permanent magnets

The figure shows four steel bars; three are permanent magnets. One of the poles is indicated. Through experiment we find that ends a and d attract each other, ends c and f repel, ends e and h attract, and ends a and h attract. Which ends are north poles?

a, c, e

a, d, h

a, f, g

a, c, f

Solution

b=S implies a=N. a attracts d implies d=S which in turn implies c=N. c repels f implies f=N. SO the answer is (iv)