The correct answer is option A, that is, the Earth is spinning on its axis.
The westerlies refer to the prevailing winds that blow from the west to the east in the mid-latitudes, that is, in between 10 and 60 degrees. The wind blows from the regions of higher pressure to the region of lower pressure, but it does not blow in a straight line.
This is generally because the Earth is rotating on its axis. The spin of the Earth makes winds to bend towards the right in the northern hemisphere and towards the left in the southern hemisphere. This is known as the Coriolis effect.