Step 1: Understand what a reducing sugar is. A reducing sugar is a type of sugar that can donate electrons to another molecule, which means it can reduce other substances.
Step 2: Identify common examples of reducing sugars. Common reducing sugars include glucose, fructose, and maltose.
Step 3: Check the options given in the question: fructose, sucrose, starch, and cellulose.
Step 4: Determine which of these options are reducing sugars. Fructose is a reducing sugar because it can donate electrons.
Step 5: Note that sucrose, starch, and cellulose are not reducing sugars. Sucrose is a non-reducing sugar because it does not have a free aldehyde or ketone group.
Step 6: Conclude that among the options, fructose is the only reducing sugar.
