(a) Magnetic line of force is defined as an imaginary line in a mangetic field along which a free or isolated north pole would move. OR, it is an imaginary line in a magnetic field such that the targent to it at any point gives the direction of the field at that point.

(b) F = 1BL Sin\(\theta\)
(i) When wire is parallel to the field F = IBL = 4x 2 x 0.1 =0N
(ii) When force is maximum F = IBL Sin90° = 4 x 2 x 0.1 x Sin90°= 0.8N
(iii) When the wire is at 60° to the field
F = IBL Sin\(\theta\) = 4 x 2 x 0.1 x Sin60° = 0.693N

(c) Magnets are stored in pairs with their poles adjacent, placing soft iron pieces called keepers across their ends. The keepers become magnets by induction decreasing the reluctance of the air gap; hence both magnets and keepers form closed loops with no free poles preventing demagnetization
.

(d) True bearing = 15 + 10 = N25°W.