The correct option is A Galactose
A reducing sugar is any sugar that 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, along with some disaccharides, oligosaccharides, and polysaccharides.
In the give options, only Galactose which is a monosaccharide sugar has a free aldehyde group.
Gluconic acid is an organic acid, β -methyl galactoside is a glycoside containing methyl galactose and Sucrose is a disaccharide composed of glucose and fructose. All of these organic compounds lack a free aldehyde group or a free ketone group that can act on a reducing agent.