See image — Biomolecules Chemistry Question

💡 Solution & Explanation

Concept: The D/L designation for carbohydrates refers to the configuration of the highest-numbered chiral center (the penultimate carbon) relative to glyceraldehyde, not to the direction of optical rotation. Reasoning: - The D/L system is a configurational descriptor based on the spatial arrangement of the -OH group at the reference chiral center. If the -OH is on the right in the Fischer projection, the sugar is D; if on the left, it is L. - Optical rotation (dextrorotatory '+' or laevorotatory '-') is an experimentally measured physical property that depends on the specific molecule and is not determined by the D/L designation alone. - For example, D-glucose is dextrorotatory (+), but D-fructose is laevorotatory (-), yet both are D-carbohydrates. This disproves options (a) and (b). - Option (c) states a D-carbohydrate is always the mirror image of the corresponding L-carbohydrate. While a D-sugar and its L-enantiomer are indeed mirror images of each other, it is not accurate to say a D-carbohydrate IS the mirror of the L-carbohydrate as a defining property of what a D-carbohydrate IS — the D-carbohydrate is defined by its own configuration, not by being a mirror. More critically, option (c) is misleading because it implies the D-form is defined as the mirror of the L-form rather than by its absolute configuration at the reference carbon. The standard and precise definition is solely about the configuration at the penultimate carbon in the Fischer projection, making none of options (a), (b), or (c) fully and correctly descriptive of what a D-carbohydrate is. - Since (a) and (b) are clearly false (D-sugars can be either + or -), and (c) is at best a consequence but not a correct standalone definition, the best answer is (d) None of these. Therefore, the correct answer is D.