Among the listed abrasives, which is the least coarse?

The correct answer is crocus cloth as it is designed to be a finer abrasive material compared to the others listed. Crocus cloth is typically treated and consists of a very fine abrasive, making it suitable for polishing and achieving a smooth surface finish.

In contrast, sandpaper is commonly available in various grits, but it typically starts off coarser and is used for sanding surfaces down. Emery cloth also features an abrasive mineral but is generally coarser than crocus cloth. Steel wool, while used for fine finishing and polishing, consists of fine strands of steel that can be more aggressive on surfaces compared to the soft, fine abrasiveness of crocus cloth. Thus, among these options, crocus cloth stands out as the least coarse abrasive.