a b c d

d

$\text{Bar magnet should require a magnetic intensity of 4000A/m to demagnetize.}$ $ H = nI$  $n = \frac{N}{L} = \frac{60}{0.12} = 500 $ $\Rightarrow I = \frac{H}{L} = \frac{4000}{500} = 8A$