In alkaline medium, fructose is

In alkaline medium, fructose is a reducing sugar.

Since ketones cannot be oxidized, monosaccharides containing them, such as fructose, are therefore thought of as reducing sugars, while the reducing sugar in reality is the isomer containing an aldehyde group. A base already presents in the solution serves as the catalyst for this kind of isomerization.

Hence, option A is correct.