Answer: Option B
:
B
Reducing sugars contain a free aldehyde or an alpha-keto group which can reduce Tollen's or Fehling's reagent. Sucrose is a non reducing sugar. In sucrose, the components glucose (alpha-D-glucopyranose unit) and fructose (beta-D-fructo furanose) are linked via an acetal bond between C1 on the glucosyl subunit and C2 on the fructosyl unit via a glycosidic bond. The glycosidic bond is formed between the reducing ends of both glucose and fructose, and not between the reducing end of one and the nonreducing end of the other, like in reducing sugars. Therefore, sucrose contains no free aldehyde or ketone group. Instead it contains stable acetal or ketal structure which cannot be opened into a free carboxyl group and therefore cannot reduce Tollen's or Fehling's reagent.