Final answer:
The finest grit sandpaper listed, which is option d. 400 grit, is recommended for the smoothest result. It's important to use progressively finer sandpaper to achieve a smooth finish.
Step-by-step explanation:
For achieving the smoothest result when sanding a surface, a fine grit sandpaper is recommended. Grit levels measure the size of sandpaper particles. The higher the number, the finer the abrasive and the smoother the finished surface. In this context, option d. 400 grit sandpaper is the finest among the listed choices and would thus yield the smoothest finish. It's important to work your way up through the grit levels during the sanding process, starting with a coarser grit to remove material and imperfections, and gradually moving to finer grits to smooth the surface.