Spokes are added to wheel A to make wheel B. Add spokes to wheel C by the same rule.

In B every spoke of A has been given a partner pointing exactly the opposite way, so each spoke becomes a straight line through the centre. C has three spokes (up, lower-left and lower-right), so the answer adds three more pointing down, upper-right and upper-left, giving six spokes: (b). (a) turns C upside down instead of adding, (c) adds only one partner and (d) has six spokes in the wrong directions.