What is the relationship between D-glucose and D-fructose?

I am not sure what you are asking, but D-glucose and D-fructose are constitutional isomers.

The structures of D-fructose and D-glucose are

They each have the same molecular formula, ##"C"_6"H"_12"O"_6##.

But D-fructose has a primary alcohol at ##"C-1"## and a ketone at ##"C-2"##.

And D-glucose has an aldehyde at ##"C-1"## and a secondary alcohol at ##"C-2"##.

This makes them constitutional isomers, because they have the same formula but a different order of bonding.

Beyond this, we can say that they are both hexoses, but one is a ketohexose and the other is an aldohexose.

Also, D-fructose is levorotatory, but D-glucose is dextrorotatory.