Chiral Carbons in Fructose Require Four Different Substituents

Steps:

• A chiral carbon is a tetrahedral atom bonded to four different groups.
• Fructose's open-chain structure is CH₂OH-C=O-(CHOH)₃-CH₂OH, with potential chiral sites at C3, C4, C5.
• X (C3) bonds to H, OH, C2 (carbonyl side), C4 (chain side)—all different, so chiral.
• Y (C4) bonds to H, OH, C3, C5—all different, so chiral; Z (likely C1 or C6) bonds to two H's, so achiral.

Why B is correct:

• B selects X and Y, the two labeled chiral carbons, as each has four distinct substituents per the chirality definition.

Why the others are wrong:

• A includes Z, but Z has identical hydrogens, violating the four-different-groups rule.
• C omits Y, but Y meets the chirality criterion with unique attachments.
• D omits X, but X is chiral like Y.

Final answer: B