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 ;-)