Upper level winds blow parallel to the contours, surface winds blow across the contour lines always toward low pressure.  So this is an example of a surface wind and low pressure must be at the bottom of the figure.  The pressure gradient force always points perpendicular to the contours toward low.  Arrow c is the only force pointing toward low pressure at the bottom of the figure and it is perpendicular to the contour.  Arrow b is opposite the wind so it must be the frictional force.  Arrow a is perpendicular to the wind (it's slightly skewed in the figure, but that's a drawing error) and is to the right of the wind when you look downstream so this is a northern hemisphere map.