The geostrophic wind results from a balance between the horizontal pressure gradient force and the Coriolis force. It is a horizontal wind, therefore, answers (b) and (d) are eliminated from consideration. The horizontal wind at the surface is also influenced by friction so that the wind there departs significantly from the geostrophic wind. Thus, the only logical answer is that the geostrophic wind is a decent approximation for the horizontal wind in the upper atmosphere; answer (a).