In the naturally occurring D series of ketoses, the carbonyl group is found on carbon number , whereas in aldoses, the carbonyl group is found on carbon number .

second carbon atom from the  end

end carbon atom

Explanation:

Carbohydrates are naturally occurring organic compounds containing carbon, hydrogen and oxygen. The general molecular formula of Carbohydrates is .

Carbohydrates can be classified based on structures,

Carbohydrates with the structure of alkanals (-CHO) are known as aldose while those of the structure of alkanones (C=O) are known as ketose.

In stereochemistry , D series is a kind of configurational arrangement where the hydroxyl group attaches itself to the right hand side.

Thus; in naturally occurring D series of ketoses, the carbonyl group is found on carbon number second carbon atom from the  end whereas in aldoses, the carbonyl group is found on carbon number end carbon atom.

the answer is option a.