Answer:
Option). They have a double-bonded oxygen in different locations.
Explanation:
Both glucose and fructose are monosaccharides, having molecular formula C₆H₁₂O₆.
Glucose and fructose are different sugars, aldose and ketose respectively, due to the different position of carbonyl group (double bonded oxygen with carbon or C = O group).
In glucose, the oxygen atom makes double bonds with last carbon atom of the chain, which means carbonyl group shows an aldehyde group. Thus, it is an aldose sugar.
In fructose, the oxygen makes double bonds with an internal carbon atom and represents a ketone group. Thus, it is a ketose sugar.
Thus, the correct answer is third option.