A reducing sugar is any sugar is capable of acting as a reducing agent because, it has a free aldehyde group or a free ketone group. All monosaccharides are reducing sugars, like galactose glucose, glyceraldehyde, fructose, ribose and xylose. Some disaccharides like lactose, maltose and ribose having free aldehyde group, so these are reducing in nature. Hence, correct option is (b).