Reducing sugar among the given compounds The question is to determine which of the following sugars is not reducing?



I have the idea that in case of disaccharide sugars  in which the monosachharide units are bonded via the carbon containing aldehydic group are non reducing.However the present question deals with sugars having monosaccharide units.I have no idea on how to deal with such sugars.Any insights?Thanks.
The English wikipedia page on reducing sugars starts with


  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 [...]


Unfortunately, the monosaccharides tend to hide the aldehyde and keto groups by undergoing intramolecular reactions between a carbonyl group and a hydroxy group. This reaction leads to a cyclic hemiacetal.

In order to figure out which of the structures in your question is a monosaccharide in disguise, we need to spot the hemiacetal moiety. With other words: 

Which of the structures can open again to show a carbonyl group?

We can definitely rule out (C)! This is a hydroxymethyl-substituted cyclohexane with some hydroxy and a methoxy group.

What about (D)? Close, but no cigar. This is an acetal, which won't open that easy.

I'll leave the last two examples to you ;-)