Try to look at the surface of the fluid as many smaller springs (represented by the velocity arrows in the answers). Each point on the surface moves up and down like a spring, with an amplitude, potential energy, and kinetic energy. Therefore, the velocity is highest at the ends of the tank because the fluid at the ends moves with a greater amplitude than any other point along the surface. Therefore, the arrows in the correct answer show the ends having greater velocities than those in the middle (actually, the very middle point won't be moving at all, theoretically).
Hope that makes some sense!