Which Fischer projection represents $D$-glucose?

• A. The Fischer projection with the hydroxyl group on the right at the penultimate carbon ($C_5$), and the configuration (from top to bottom, $C_2$ to $C_5$): right, left, right, right.
• B. The Fischer projection with the hydroxyl group on the left at the penultimate carbon ($C_5$), and the configuration (from top to bottom, $C_2$ to $C_5$): left, right, left, left.
• C. The Fischer projection with all hydroxyl groups on the left from $C_2$ to $C_5$.
• D. The Fischer projection with alternating hydroxyl groups (right, left, right, left) from $C_2$ to $C_5$.

Answer: (A)

Verified step by step guidance

1. Recall that the D- or L- designation in sugars is determined by the configuration of the hydroxyl group on the penultimate carbon (the second to last carbon in the chain). For D-sugars, the hydroxyl group on this carbon is on the right in the Fischer projection.
2. Identify the penultimate carbon in the glucose molecule, which is carbon 5 (C\_5) in the six-carbon chain of glucose.
3. Examine the configurations of the hydroxyl groups on carbons 2 through 5 (C\_2 to C\_5) in the Fischer projection. For D-glucose, the pattern from top (C\_2) to bottom (C\_5) is right, left, right, right.
4. Compare the given Fischer projections to see which one matches the pattern of hydroxyl group positions (right, left, right, right) from C\_2 to C\_5 and has the hydroxyl group on the right at C\_5, confirming it as D-glucose.
5. Remember that the other options with different patterns or with the hydroxyl group on the left at C\_5 correspond to L-glucose or other stereoisomers, so they can be excluded.