Glucose and fructose both have the molecular formula C6H12O6. Are they isomers, polysaccharides, oligosaccharides, pentoses, or steroids?

The question states that glucose and fructose both have the molecular formula C$_{6}$H$_{12}$O$_{6}$, but their atoms are arranged differently.

• Isomers are compounds that have the same molecular formula but different structural arrangements of atoms. Glucose and fructose fit this definition perfectly; they are structural isomers.
• Polysaccharides are large carbohydrates made of many monosaccharide units. Glucose and fructose are single monosaccharide units.
• Oligosaccharides are carbohydrates made of a few monosaccharide units (typically 2-10).
• Pentoses are monosaccharides with five carbon atoms. Glucose and fructose have six carbon atoms, making them hexoses.
• Steroids are a type of lipid, not carbohydrates.

Therefore, glucose and fructose are isomers.

The correct option is:

• isomers.