Fructose and galactose both have six carbon atoms, twelve hydrogen atoms and six oxygen atoms, just like glucose. a friend of yours says: "since those sugars have the same number of atoms, they should have the same name." you "while they do have the same number and type of atoms

Answer is: they are named differently because they are alternate forms (isomers) of the monosaccharide, glucose.

An isomer of a molecule has the same number of atoms of each element (in this example, glucose, galactose and fructose have molecular formula C₆H₁₂O₆), but has a different arrangement of the atoms.

Difference between D-glucose and D-galactose  is the orientation of the hydroxyl group (OH) at fourth carbon atom, at D-glucose -OH group is right and at D-galactose is left.

Difference between D-fructose and D-glucose is that first one is ketonic monosaccharide and second one is aldose (carbonyl group on the endmost carbon atom).

the answers would be

the first one and the second one